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?
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.