Nirvana Fallacy

The Nirvana Fallacy occurs when someone posits an apparently perfect solution to a problem that sounds great on paper, but in reality overlooks or ignores other factors that would most likely come into play and prevent it from actually working out as planned. In essence, this fallacy is about assuming perfect and ideal conditions that don't actually exist.

For example, Linda decides that she is going to make her daughter into a polymath by having her taught multiple languages, how to play multiple instruments, how to paint, and all about various fields such as astrophysics and philosophy from as soon as she's old enough to talk. This ignores the likelihood that her daughter will probably want spend time doing things besides learning the subjects her mother has chosen for her, and what's more it's extremely likely she'd have neither aptitude nor interest for all of them.

Or, Will decides that he's sick and tired of all of the religious bickering in the world, so he up and creates a new religion/philosophy to try and unite everyone. As soon as everyone's converted to Willism, he decides, then all shall be perfect and wonderful. This ignores the fact that while it's usually easy enough to find some people who are willing to switch to a new religion or philosophical system, converting the masses is another matter entirely. Those who are content with their current belief systems would have no desire to change, and others would find parts of Will's new system flawed or even repugnant. (The actual end result would be that Will's little movement would end up competing and bickering with everyone else, thus becoming part of the problem he tried to end in the first place.)

This is closely related to the Perfect Solution Fallacy.