I've been thinking about some of the game's mechanics, and I have a few suggestions.
First of all, guns currently don't have very many customization options and are fairly basic. It would be nice to either have more hardcoded attachment slots for things like stocks, barrels, handguards/frames etc or have a way for mods to add new attachment slots.
Additionally, there is currently no way to make guns able to use multiple different magazines and/or ammo types without complex workarounds; for example, if a mod wanted to add an assault rifle that can take either normal or extended magazines, it would have to change the gun's data every time it switches to a different magazine size, and as far as I know there is no way to make different varieties of ammunition. (Hollow-Point, FMJ, AP etc) Some sort of fix for this would open up a lot of possibilities.
Finally, there currently isn't an easy way to make a mod that replaces in-game sounds or animations. Most mods that do this have to manually replace files to work, which causes several issues and inconveniences. A function that lets mods more easily replace these things would be great.