It can be improved as much as you want, however that would require a person to modify the game code.
Ironically, despite the popularity of wc3 in the modding community there have been virtually no graphic mods for it. (compared to things like Morrowind for example)
We could blame that on two things, one being that it is predominantly a multiplayer game.
Tampering with the source code could cause your game to not work online or in worse case cause your battlenet account to be suspended because such modifications are very likely against the terms of use.
Secondly, online maps have 8mb filesize limitations. You can do very little with that much space pretty much limiting anything past that to single player or use of local files which has similar drawbacks as the first reason.
There is also the problem that everybody would have to download such modification for it to work.
That being said, if filesize is not an issue you can fake many things.
You can fake physics, shadows, illumination and other things by creating such animations/textures manually on a per-model basis.
Things that would require to modify the game obviously can't be done, but you can do/fake a lot of what current generation games can do.
Obviously it would take much more time and effort than using pre-made assets, unless you rip assets from modern games, lots of them.
