Folding vehicles
It's fairly rudimentary right now, I added a "folded bicycle" item that when activated spawns a regular bicycle at your location. To keep the code simple I disallow modifications to the resulting bicycle except changing the tires and repairing components. The resulting bicycle has an extra vehicle menu entry that folds it back up into the item. They spawn... oops, nowhere, need to fix that. Now that I think of it I should also add a crafting recipe to make the thing. It weighs 20lbs and takes up 80 volume (whatever THAT is), so it's extremely unweildy, but not too big to throw in the trunk of another vehicle.
At some point I'm going to refine it to serialize the list of components so that you can make modifications to it that will be persistent across folding/unfolding, the current version only saves the HP of the components so you don't get a whole new bicycle every time you do a fold/unfold cycle on your.. cycle.
Eventually I'll see about expanding this to handle arbitrary vehicles, so you can have a little fold-up go-kart you stash in a trunk or something, will probably require you to use "folding frame" components or something to link things together, and have a weight limit.
Giving players tools
Yea, this is how I think of it, I try to make the various things I add make sense and interact with each other properly, and then players go nuts with it and do unexpected stuff, and if the interactions were structured properly, cool things happen.
Falling
Actually we'll be tracking terrain/furniture/monster height for stealth purposes <_<
So sub-tile resolution might get factored in, but really that only matters if you've only fallen a tile or two. Falling 2.5 vs 2 tiles is one thing, but the results of falling 5.5 tiles vs 5 is going to be indistinguisable. And there really isn't a "collision system" per se, when you get flung from a car or by a explosion or monster, it does a set of calculations, but I'm not at all sure they're sensible in any way. I think the more interesting question is going to be what you fall on. Definitely have to make sure to get falling onto the roof of a car... right.... cars will have roofs... that you can stand on. Have to keep that in mind.
Not too many flying monsters everywhere, I hope. The thing about flying is, zombies can't reach you, which is important. A few 'gliders' or 'jumpers' would be decent, but there shouldn't be so many that flying becomes dangerous. Floating over the horde should still be a viable way to survive.
At the same time, ant nests, wasp nests, and large cities should have enough flying critters to make travel difficult, because certain high-danger areas should become no-fly zones.
Jumping spiders 4 feet tall? Now that's going to be a long jump. 3-4 vertical z-levels worth.
Who said it was just going to be up? Giant spiders and their giant webs...
But yea, mostly I think the main danger you'd experience when flying is... flying.