advowson: I think there are some technical factors that work against the creation of such a mod. If the mod runs on the original engine and only replaces the monster sprites, then it has very little room in which to exceed the original game. There is only so much detail that can be packed into the small viewing area offered by the original engine, especially since it would also be constrained to use the simplified color palette of that era. Using more screen space than the original engine offered would probably be a big undertaking, and would be much easier if the game were reimplemented outside the original engine. ScummVM picked up a Xeen interpreter in the last few years; that might offer a more flexible way for a graphics mod to replace content. The next problem would be that if the original sprites cannot be programmatically up-scaled/adapted to the new display engine, then either the mod would need to replace all the monster sprites, or some sprites would look awkward. Finding someone willing to draw replacements for some sprites is hard enough. Asking them to replace every monster sprite would be worse. The whole problem gets worse still if you want to replace any of the backgrounds or sound effects.

Finally, there is the question of why one should bother. In my opinion, if you're going to create new content, it should be content that either makes the original more accessible (e.g. translating to a language that the original engine was not released in), makes the original more usable (e.g. changing some items to say what they mean in-game, without needing constant visits to the Blacksmith), or adds areas that veteran players would want to play, such as by adding new dungeons that no one has seen before. I wouldn't consider a pure-graphics mod that changes how World of Xeen looks to be a reason to play it again, since I know the existing campaign. If I was loading a mod, I would want a new campaign.