"Is This A Good Idea For My Story/Setting/Character?"
How To Answer This For Yourself!
People ask these types of questions every day - but they're not always so simple for others to answer. People don't always have the time to analyze your ideas in-depth, and they don't have the benefit of being inside your brain to understand and analyze your idea in its full and complete context. So sometimes, the best way to figure it out is to... ask yourself!
While you might still need to ask others some questions afterward, starting the process by asking yourself the right questions can save you and others a lot of time and energy. So let's go!
How will your target audience likely react to it? Think about the type of people you intend to reach with your work. Are you trying to write a story for children? They probably won't appreciate a multilayered plot about a corrupt oil company. Are you trying to reach an older audience looking for something fairly suspenseful and serious? They'd very likely be annoyed by a bungling/cowardly sidekick-type character. Did the work originally gain a lot of fans because people enjoyed the way the characters played off of each other? You'll probably lose these people if you shift the focus to fast-paced action instead. Try to make your creation the kind of thing your intended audience will probably like.
How would you react to it if you saw it in someone else's creation? Would it get you interested and engaged? Or would it more likely bore, confuse, or annoy you? If it would be the latter, it's probably not a very good idea. So think about what it would take or how something might be done so that it would get you interested in it, and try working something out from there.
Does it fit the setting? In a setting where supernatural creatures typically range from a hassle to deal with to downright deadly, a cheerful magical fairy who grants free wishes with no strings attached popping out of nowhere would most likely seem jarring. In a world where everyone who has been affected by strange mutagen have all ended up with bizarre, even creepy features, someone else ending up with angelic wings and cat ears would feel out-of-place. In a lighthearted series set in a small town where most characters are mostly friendly and the more unpleasant members of society rarely get up to any serious trouble, a sadistic serial killer suddenly thrown into the mix would probably be overdoing it. Unless you're deliberately trying to create a dissonant atmosphere your idea should jive with the general setting you're working with.
Does it fit with the way real people act and react? There are far, far too many examples to go into full detail here. Minorities are not hive minds. Taking digs at things people are sensitive or passionate about is how you make people hate you, not love you. Same goes for being presumptuous and bossy. Normal parents do not sit by and twiddle their thumbs if they believe their children are at risk. The government does not cover things up for being mildly unusual. Just because someone is evil in one specific way does not mean that person must be evil in every conceivable way. No competent leader would knowingly send a bungler on a high-stakes mission if there was any other choice. Killing someone for the first time doesn't flip a moral switch that turns one into a compulsive killer. Unless you have a deliberate reason for people to be acting strangely, try to keep close to how people would act in reality.
Does it involve an extreme or excessive element in some way that isn't strictly necessary? Be honest. Does the story you're trying to tell require that your character be The Only X In The Known World? Does your character really have to be The Most Gifted Y Of Our Times? Is it really necessary for your protagonist to be the Actual Undisputed Most Gorgeous Person At School, rather than simply being reasonably attractive? Is it absolutely essential for some reason for your character to have personally witnessed an assassin brutally murder the entire family? If you can tone something down a few shades and still have it work out, you probably should.
Does it add depth or nuance to the setting or to a character? Will it help the audience better understand or appreciate something relevant to the story? Will it help make a character's actions or choices make sense to the audience? Will it show why the something relevant to the plot, be it a person, place, institution, etc. is the way it is now? Will it show us something important about the way a culture lives, thinks, etc.? If so, it might be a good idea.
Will it open up new plot potential? Will it provide or lead up to new places to see and new things to discover? Will it lead up to meeting characters who will become important to the plot? Will it open up or create new opportunities for the characters to end up in dramatic scenarios that you haven't done already? If it fits into the tone and style of the work and opens up new plot potential, it's probably not a bad idea.
Or will it close off more potential plots than it opens? This is something you really have to watch out for - some elements might open up a few potential plot threads at the cost of closing many more of them off. Granting a protagonist godlike powers might make it possible for the character to finally take down other super-powerful characters, but without putting careful thought into it you run the risk of making it impossible to challenge the protagonist enough to make a dramatic storyline later on. Anything you're considering adding in, try to make sure that it doesn't destroy large swaths of plot potential before you're ready to end the story or retire the character.
Or might it have any other implications or logical consequences that might be unwanted or troublesome? For example, if people realize that your bad guys are killing so many people that there shouldn't even be a sustainable population left by now, they may lose the ability to suspend their disbelief. Someone inventing a cure for any illness that could easily be made with a few things one could pick up in town would have some pretty big consequences for the healthcare industry and its workers - would you be willing to sort that all out and figure out how things settle? If 10% of the whole population spontaneously develops superpowers, that's going to be one in ten people. Now consider how many people are in your school or workplace, and just how many people 10% would turn out to be - are you sure you'd want that many supers running around? If your idea would have implications or logical consequences that might take the setting or plot in an unwanted or troublesome direction, you should probably adjust it.
Or does it add needless complexity to an already-busy storyline? Another trap to be aware of. If you create so much complexity or get so much going on at once that you have to downplay or shelf arcs and plot threads that were a main focus before, you may have a problem. If people are interested in seeing what you've already got going on play out and resolve, they're going to end up frustrated and disappointed when they realize that you're not going to give them what they want anytime soon. Note that if your ideas pass the other criteria in this article, you can keep them in reserve for later use.
Does it contradict something you came up with earlier? Whenever you're coming up with a new idea for your story, stop and make sure it doesn't contradict something you came up with earlier. For example, if you decide that your vampires burst into flame upon stepping out in the sun, did you earlier show them stepping out into it with no problem? Or does vampiric history somewhere contain events that would have almost certainly relied on vampires being outside in the sunshine? If you run into a problem like this, you either need to change what you've already established to fit what you've just come up with, explain why it's suddenly different now, or just leave it the way you originally established it.
Does it pass a "why didn't anyone just do this one thing?" test? Ask yourself a few questions, as they apply to your work: Are there any simple actions that someone in the story could have taken that would have averted a whole lot of pain and suffering, or at the very least have made things a lot simpler and easier? If so, why weren't these actions taken? Is it really that likely that nobody got curious or suspicious enough to start poking around or call the cops? Why didn't anyone just ask something like "hey, what's wrong?", "why do you believe that?", or "why do you feel that way?" If you cannot provide reasonable answers to these questions, your idea probably isn't very good.
Does it pass a "why didn't anyone get up and fix this problem already?" test? Is it really that likely that nobody in the history of ever would have worked up the gumption to do something about the source of the central conflict? Is it really that likely that nobody would have thought of some solution somewhere? One exercise you can do is to put yourself in the shoes of the people or factions who would have been adversely affected by the source of conflict for awhile, and assume that they want to do something to fix the problem as much as anyone does. What do you imagine them doing about it? Assume that's what they would have done. Would they have actually stood a pretty good chance at fixing the problem, given what you've come up with? Then you either need to have people fixing the problem a lot sooner, or come up with some good reasons why they didn't.
Has it been done often before? If so, are you willing to put in the extra effort it'll take to make it new again? Go onto any site you might put your work on and do a search for similar stories or characters. Do you see many of them using the same concept/concepts as what you're considering? If so, are you willing to look through them and figure out for yourself to figure out how they've all been done so you can figure out how to do something different with it? People need new experiences to be interested and entertained.
Have you let it simmer? New ideas can seem awesome when initially conceived, but if you let them rest for awhile and examine them later on, you'll often find they aren't quite as good as you thought they were. Write your ideas down, let them rest for a few days, then come and look them over again and see if they hold up. This goes extra if your idea came from a dream - something can seem like utter genius when you first wake up, but give it just a few hours and you'll often find that they seem utterly absurd. If you find your ideas don't sound quite as good as they did earlier, ask yourself what you might change, remove, or add to improve them.
Before you go, don't forget - your final product does not have to be 100% "perfect." It's all too easy to get caught up in an endless loop of fretting over whether each and every little thing is absolutely perfect. But the truth is that no work is entirely and objectively perfect, and that's okay. What you should do is strive to make a good product and try not to get too emotionally attached to it. Then, if it does badly, try to learn from your mistakes and do better in the future. There's always tomorrow to try again and make a fresh start!
Also, you might be interested in:
How To Break Your Creative Blocks
Writing Better Prompts, Starters, & Beginnings: A Few Pointers
On Plot Structure & Plotting
Plot & Story Development Questions
Stuff You Should Cut From Your Story
Reasons Your Story Might Be Stuck - And How To Fix It
On Showing vs. Telling
Describing Your Character: Tips & Advice
Basic Tips To Improve Your OCs & Fan Characters
Basic Tips To Make Better & More Appealing Roleplaying Characters
Character Development Questions
Borrowing & Sharing Ideas In Fiction - When It's Okay, And When It Isn't
Setting Rules & Limitations In Your World: Why & How You Need To Do This
Things Your Fantasy Or Science Fiction Story Needs
Things You Need To Do In Your Science Fiction Or Fantasy Story
Tips For Writing Fanfiction With An OC Protagonist