"Should I Add Or Allow This In My Game?"

Does it fit the game's setting and tone? If you're running a roleplay based on Star Trek, you might accept a character who is a member of an alien race the player made up, so long as it fits the spirit of the show closely enough that you could easily imagine them featuring in an actual episode. On the other hand, a character who is supposed to be an actual angel from actual Heaven? What with this being a franchise where the existence of divine forces is left at best ambiguous or open to interpretation, you'd likely want to turn that down.

Would allowing it create a lot of work you'd rather not get into? If, for example, the creation of a new race would require you to make an entirely new template and you'd really rather be running the game than doing that kind of work right now, then it's fair to turn the idea down - or at least delay it until you're ready to spend that kind of time.

Will it potentially imbalance the game, or is someone likely to abuse it? Will a power or ability that someone wants for a character make that character pretty much invincible against everyone or everything else? Or will it make the character able to solve pretty much anything, leaving everyone else with nothing to do? Then you may want to restrict it, unless you're very sure that you can trust the player not to abuse it. Or you might just want to restrict it period, so that other players don't think you're playing favorites. Or you might allow it - with the stipulation that some sort of limitation or flaw be added to it to keep it from becoming OP - see Setting Rules & Limitations In Your World: Why & How You Need To Do This for tips and advice on this.

Is it likely to break the game in some other way? Can it quickly and easily remove every major source of conflict? Will it make it difficult, if not impossible to add in new sources of conflict? Will it leave the PCs without the resources they need to carry out their business? Will it leave them unable to do much of anything besides interact with each other for any other reason? If so, then you most likely don't want to allow it.

Is it likely to disrupt an ongoing plotline? If the players are already merrily solving a mystery, now is probably not the time to launch an alien invasion. If they're in the thick of fighting off an alien invasion, now is probably not the time to have somebody's little sister get kidnapped by terrorists. If they're preparing to rescue somebody's little sister from terrorists, now is probably not the time to have a wizard drop by and turn everyone into koalas. Keep new ideas for when players actually need something new to do.

Will it help facilitate new adventures and plotlines? If you never add any new characters or elements, it's going to eventually become impossible to do anything new, which will make game eventually grow repetitive and dull. Allowing the addition of new elements and characters, even if they aren't strictly canon, can help keep things fresh and interesting.

Does it outright contradict the game's own canon? Does it contradict something that happened earlier in the game? Does it run contrary to the setting's backstory? Does it rely on an NPC or background character behaving uncharacteristically? Does it fly in the face of how the setting's cosmology is established to work? Then you might not want to allow it.

On the other hand, is there wiggle room for changing the canon? Maybe some bit of canon or other has never actually made an appearance in the game itself nor has ever directly affected what's going on in any significant way. If a proposed change wouldn't affect anyone's character, nor would affect the game's setting or its story up until now in any serious way, then you might consider allowing it.

Would it have any undesirable consequences or logical implications? For example, destroying the PC's home base might make for an interesting scenario, but would doing this leave them without a place to work and/or live? If so, is that something you and the players really want to deal with right now? Revealing that your characters' enemies are really in charge of everything could perhaps add some momentary intrigue, but then how do you account for the times when people in fairly high-ranking places helped them in ways that their enemies surely would have not wanted? Would a new power or plotline likely contribute to dramatic hyperinflation? If something would likely have consequences or implications you don't want, then you probably shouldn't allow it.

If it will affect or attract the attention of the PCs, will they be able to do anything about it? Stuff that the PCs can do something about is stuff that creates plot potential, and that's good! On the other hand, stuff that they can't do anything about does not - about all it's good for is annoying players who don't appreciate watching things they can't do anything about being waved around in front of their faces.

Will it put any of the currently-present players on the bench? Anything that leaves any attending player with no possible way to contribute to the plot (unless that player has given consent) probably isn't a good idea. It's rude and unfair to exclude players thus, and you risk annoying them into leaving your game entirely.

Is it likely to cause someone in the game distress? If it involves subject matter that you or a player finds especially uncomfortable, it's all right to disallow it, or at least the depiction of it in the game.

If you're inclined to say no, are you perhaps just being too finicky? If it passes all of the above checks and yet you're still inclined to deny it, it's possible that you might just be an unreasonably finicky GM/RPA. Remember, forbidding things for no good reason as far as your players are see will frustrate them and may even drive them away from you and your game.

Is your judgment being affected by a power-trip? Anybody can end up power-tripping without even realizing it, so it's always good to try and watch yourself for signs that you're doing this, especially when you're trying to make a decision as GM/RPA. Check out When A Game Master Or Roleplay Admin Might Be Power-Tripping - And What To Do About It to familiarize yourself with some warning signs. If it might be that your judgment is being affected by a power trip, then you probably should not try to force things your way.

What do your players think? If you're still unsure, ask your other players what they think and what they want. Being a GM/RPA is ultimately about making your players happy, so make a habit of asking them what they want or whether they have any suggestions, and take them into account. Remember, trying to force things that your players don't actually want will make you look egomaniacal, and constantly forbidding things that they all want or think is fine will make you look petty.