I'm no game developer, but wouldn't creating new higher-resolution textures, and more textures, and more textures, as well as particle effects take a lot of... time?
It would be cool and all, but it sounds like the game would have to be dismantled and rebuilt from the ground up.
I was wondering the same thing about the water ripples... I always loved that
Depends on how badly it was programmed. If it was modular then you only need to tweak or rewrite the graphics engine itself and leave everything else alone. As for textures they could hold a contest for us to create textures for different things following various rules of size and file types. Doing that could retexture the whole game quickly and easily. Nothing like crowdsourcing for speeding up a task. There are many on here that could do that easily. Overall it could be done fairly quickly with little effort over the long run as the community would carry the heavy texture load.
Just look at games like Skyrim to see how much improvement the community can add to the graphical experience of the game.