Hmm. That's a tricky one, because dreams are rather broad-reaching. I'm not much for sims, but I like sandboxes.
I guess I'd want something set in either a modern or future-esque world with a BPRD/Dresden style twist. Noir-like mysteries, monsters go bump, magic hangs in the corners, and dark gods await summoning. I think leaning more towards Dresden-y stuff would be my preference as I would really love to see a good magic system at work in an open world environment. How "known" magic is to the populace and whether or not the game uses the future version of the "it's just like our world, except somehow no-one exactly noticed magic properly" conceit I'm not sure. It could work either way. I'm leaning towards yes, because as odd as it can seem from a gods-eye-view, it's a lot of fun down on the ground, in the middle of a plot arc.
What I'd like in particular is a magic system that captures what that of the Dresden RPG and indeed Magicka did--how easily one can call down too much power and screw up. I want a magic system that surprises, that has negative consequences related to BEING a spell-caster as well as the actual casting of spells (soulgazing and hexing of electronics being the big ones from Dresden, but we could make new ones or work out something other than hexing if we want wizards to be able to use space travel). I want to be able to play it safe for some reasonably fun effects, and then push my luck to try a weird combination of things or draw in more power than I can normally handle.
I like the one, big, detailed city approach. A story that takes place over time, AC2 style would be really cool--not necessarily that extended, but enough that the player gets to see the weather change and allow for some neat environmental touches--a building undergoing construction, places open and close, damage caused by the player or other entities gets cleaned up (at least a bit). Crowds would react sensibly to hell raising. Too much mucking about would make people start recognizing you as a criminal or troublemaker; changing clothes or otherwise changing appearance would be an important part of remaining incognito when you get into dangerous turf or become notorious, and getting into trouble with law enforcement would possibly result in arrest rather than death or a trip to the hospital--unless you decide to force that sort of thing (of course, that would mean there'd have to be in-fiction ways you're character could get out of an arrest, possibly tied to a bigger conspiracy/noir plot at work or that arrest is just a fancier AI at work before your Game Over screen).
I'm not sure if I'd want it to be first or third person. Either way I'd want to make use of some of the great movement systems we've seen in games. Characters that can leap from building to building, run like Faith or Ezio ... I want to see some of these cool movement systems integrated and played against each other. There'd be flying cars to play with, which could only make it more awesome. This is probably the most important part for me; a game with a good method for getting around can really draw me in. I'll just wander around aimlessly, set my own goals, and have a lot of fun racing about. Spiderman 2 seems to be a crowd favorite for this sort of thing.
Failing all of that, taking the general design principles and movement principles of Mirror's Edge into an open-world, sandbox context would be amazing.