They've mentioned a couple of times during the campaign that there won't be any respawning monsters/animals/people/etc. (iirc. Swen detests it), only healing (at a speed yet to be decided) of hurt ones you didn't finish off.
I *think* they'll periodically restock store supplies - and maybe even replace personal items - in a sort of mini-simulation of the economy, though. (I feel reminded of the one "quest" that got mentioned a couple of times in which if a particular N.P.C. doesn't get his sword back or any other sword from the P.C.s within a certain amount of time, he'll just go buy himself a new one. - Maybe he doesn't have enough money for it if you don't give him any, though? It's been a while.)
Hmm... are there N.P.C. hunters doing their job? If so, normal animals may respawn, I guess.
Personally I think it'd be cool if you could totally screw up the ecosystem (like apparently one theoretically could in vanilla Ultima Online), but the next best thing is that if you kill it, it's dead and can't reproduce. (Not that I'd expect any reproducing going on - aside from potential special events - anyway, no matter how many nights you rest in-game.)
If you want more dungeon diving, well, that's what user-created content is for. Same for mundane player-housing. Seems easy enough to me. Dunno, you may even be able to get a key to a house of your own during the official campaign. I'm perfectly happy - ecstatic, even - with the Shelter Plane at the End of Time, though.
And I'm not 100% sure, but I think they've said there'll be a valid story reason for not allowing you to continue after you beat the final main quest. Might be confusing it with another game, though.
