My discworld proposal ("Ook!") may turn out to be too specialised, but it has currently more followers than the fantasy proposal but would make a good subset using a [discworld] tag. So should I close it or leave that decision to the followers? I mean, does that number indicate the desire for a more specialised site or may they just not know about the more general one?
Same question for The Simpsons VS Animated Series.
There is no single correct answer to this question. Some proposals are too broad (extreme example: "Everything," AKA Yahoo! Answers) while other proposals are too narrow (extreme example: "Sonic the Hedgehog, Green Hill Zone, Act 2"). The system is designed to let people naturally build communities of things that are the right size (not-so-extreme example: "Gaming"). Worst case scenario, some tinkering can happen during or immediately after beta.
In your specific case, it might not hurt to cross-advertise a bit between Discworld and Fantasy, but I suspect that Fantasy is a bit too broad and Discworld is popular enough that it'll be able to support itself.
I definitely wouldn't pull a proposal out from under a large number of followers; in addition to making them angry and confused, it'll probably be ineffective, since someone will just repropose it.
