I'd like to present the ZDoom mod I am working on, replacing monsters on any Doom/Heretic/Hexen map with Might & Magic 6,7,8 ones. The goal of the mod is to complement 3rd party RPG/fantasy mods, adding immersive features like faction-based infighting and elemental resistances. Main feature of the mod is being able to pick one of three factions you want to fight against, either in menu, or randomly every time the map loads, complementing the wide choice of player classes already present in gameplay mods.
Here is a chart showing every monster in the mod:

Dark Faction is an alliance of Deyja, Eofol and Nighon as seen during Restoration of Erathia. Order Faction consists of knights of Erathia, elves of Avlee and wizards of Bracada. Finally, Chaos Faction is barbarians, pirates and outcasts hailing from Krewlod, Regna and Tatalia. Colors of squares mean individual towns in HoMM3 (including Conflux) and element colors mean Physical, Poison, Electric, Ice and Fire respectively.
I believe the sprites in Might & Magic VI-VIII games are of insane quality, and they seem to go incredibly well with what ZDoom engine is today, making this mod an eye candy by ZDoom standards. And you can further modify your experience using a plethora of UI mods, graphical options in ZDoom engine itself (postprocessing, etc) and sophisticated modern map packs squeezing every last drop of performance out of it.
The mod itself is not available now, it's too rough, not all of the sprites are even imported yet, projectiles need work, compatibility needs work, monster behavior needs A LOT of work. But I expect I'll be able to jury-rig it together in a few months time and release a full-feature playable version that I'd be able to polish further.
Here are some early gameplay videos captured on LostCivilization WAD with two very different gameplay mods: Walpurgis (brutal fantasy mod that is all about cool powers and weapon feel) and Wrath of Cronos (the mod about progression, builds, RPG systems).
And the latest (sorry about the length) video has Bloom on, making light sources and projectiles look oh-so-pretty: