Nowadays, I mostly code out of greed. Having some success getting some ad revenue, I try to figure out ways to get moar. However there are some things which I would like to create just for fun. One of them is adding the Another World gun to the game Half-Life 2 Deathmatch. The illustration below of how the gun works is lifted from the author Eric Chahi's page for the Another World game.
Another World (called "Out of this World" in USA) was one of the first, if not the first 2D computer game to use polygonal graphics. It was also remarkable in that the very graphical game was drawn and programmed by the author alone, outside help was only used for the music. It is rare to have both artistic and programming talent in the same person, although that may be somewhat a self-fulfilling prophecy as because of this belief programmers might get a bit discouraged from even trying. Well, in any case the gun works such that pressing quickly fires a thin ray, holding button down for a moment and then depressing creates a shield, and holding for a long time and depressing creates a huge burst which can also destroy shields. Shields can be walked through, but thin rays cannot pass them. This enables you to extend an arm from behind the shield to shoot from safety.
I imagine in 3D this would work very similarly, except of course the shield would be a plane, extending a limited amount. For artwork it could use the shields already in the game, as seen in the picture above. So what's preventing me from adding this? Well, since it's a gun I think it could not be just a server modification, and it seems like overkill to release a client-side mod just to add one gun to it. If I did that, well likely nobody would install it, and nothing is sadder than a nice mod nobody uses. I've seen some great mods that added really nice weaponry like flame throwers, but nobody was playing those mods. Now that leads me to the next suggestion.
Multiplayer deathmatch games like this should have a plug-in system, where servers could push sandboxed client-side code snippets to the players. This would create an incentive for server owners to create new stuff and weapons, since it would attract more players to their servers. It would also be good for the players, because then we'd have all that new stuff. Sort of like Second Life, except with an engine more suitable for combat. The controls and "gameplay" feel in Second Life are horrible, so it certainly doesn't fill this need.