I completely agree on the factions, prison, and minigames opinion. And to answer your question about vanilla survival, I'm talking about pretty much an anarchy server. As if you were playing single-player, but with many other people.
I would consider doing a modpack, though I do know it's harder for people to join those usually.
I'll be completely honest here, you basically just read my mind. In fact, this custom idea is basically what I've had in mind for quite awhile now! I really appreciate the input and amount of detail. Perhaps this is something I'll do with it very soon, we'll see.
Well, if I was going for a certain theme, I would probably go for a mix of medieval, and also fantasy. It would include things such as races (i.e. elves, orcs, etc.), classes (archers, battlemages, etc.) and all of that other good stuff. The builds would like I said, be more medieval-styled and kingdom-like.