AOS engine could support sprites (in GIF and APNG formats) like in Skulltag Doom/Zdoom engine. It would allow to add cool stuff to maps, like plants, small rocks, spiders, fire, balloons, lamps and whatnot. Graphics files maybe should be limited in size/ammount of colors/something so they don't eat up all the traffic and map files don't become fucking huge.

For the good of client side modding - I'm thinking that the game could have an option to use animated sprites for weapons/tools. At least this is somehow possible with weapon scopes now, but ability to replace weapon model with multiple sprites for player's 'local' viewpoint and APNG (animated PNG) & GIF support for them would be nice.