A new feature that I would love to see would be to change sword and handgun to melee and projectile.
While swords already are melee weapons and handguns projectile weapons, I feel that weapon choice could be so much more varied and allow more visible expression if we aren't just limited to swords and guns; why not include such choices as warhammers, crossbows, polearms, tasers, brass knuckles, boomerangs, etc.
I'm not saying OOO should go crazy making as many battle-ax variants as there are sword variants, just make a couple of alternatives available.
It would even be relatively easy to implement: just change a few wordings here and there and make the models, textures, icons, some new attack animations, and coding for the weapons, plus some balancing. Which should still be easier than messing with core gameplay mechanics.
(I know I've neglected to mention bombs, if anyone has any ideas on other types of explosives that would look visibly different from current bombs but not deviate excessively from current bomb mechanics feel free to post them.)