Funny. Vehicles is something that I'm not looking towards. I mean, I'll take it gladly, but I was ok with the bike -- then again, mine never fell through the world.
I'd like something to do at higher levels, while I work on bigger bases. Looting isn't fun because the loot is useless, and there just isn't enough (or interesting enough) challenges.
But, most of all, I'd like improved zombie AI. The breadcrumb system looks interesting, but the fact is that I'll take anything that makes the zombies more challenging outside increasing their damage, their hp or their number.