I actually took a look at doing this after doing the textures over... it isn't that huge an amount of work as there aren't that many frames per weapon, but it would be kind of a pain keeping the colors and lightsource consistent.
Doing the weapons in 3D a la M:R would be the ideal (by which I mean rendering them in a 3D app, then making 2D sprites from the renders).
Pretty much the same deal with the sprites: I'd say making 3D models of the various monsters and taking screenshots of the walk, attack, and death animations would be the easiest way to replace the sprites. In fact, no reason why someone with M:R and maybe 3DSMax couldn't do that now...