First, woot.
Second, waiting on that windows port that someone will inevitably do.
Read my post, or the linked thread? Aposos is already working on it.
Redo every weight and volume value to be based on the smallest object in the game, and to have item stacks calculate weight/volume appropriately based on the number of objects in that stack... If they are not already doing that. In pre-construction, you could actually have "Bandages (1)" and "Bandages (2)" but their weight/volume would add up to 2/2 even if both stacks were originally "Bandages (3)", or even if you reduced the (2) stack by 1. (the weight/volume should be 1/1 or less)
I concur with this. Increasing the granularity of volume/weight shouldn't be problematic, and refactoring it such that everything has positive weight/volume and stacks are calculated properly (if they aren't already) is a good idea.
Following that premise of making inventory make sense, maybe also redo/fix the values and math behind wearing multiple clothing layers. Maybe just increase the granularity; it's really strange: I can wear a baseball cap and then put on a baseball helmet, and get 4 head encumberment... But then if i try it reversed by wearing a helmet first and then put on the cap, i get nothing.
Accessories could use some work, too. There's one type of holster that's basically useless at this point, since it goes on your leg, meaning it layers with pants, meaning you might as well just wear a pair of pants instead. Also, fanny packs being infinitely layered on your person is kind of amusing but silly.
Oh, and a volume indicator. Yourself and your environment. Maybe implement hearing protection so you don't go deaf from all those grenades detonating just a few tiles away, and shooting bursts from high-powered automatics.
Speaking of indicators, I'd love an indicator for how long it's taking you to perform actions, sort of like the one you get in DC:SS with real turns turned on. This way, it would be more clear how long it's taking to perform certain actions, or even walk.
For that matter, while the topic is random suggestions, I'd like to see the detailed stat descriptions from the chargen screen in the regular character screen as well, and max HP to be properly updated if your base strength goes up/down in game (mutations, purifier).