I like all of these ideas, but to compromise with the work that puts in, maybe we should just ask Mojang if they would update nether fortresses. Those already satisfy the requirements of a maze and a fort and have many unfinished rooms, roofs, and balconies, that could easily have space justified by red dragon mini-bosses, nether-tree farms, alchemy shops, etc.
I've said before that the problem with structures isn't that there are few too types or too far apart, but that the existing ones are not used to their full potential. It's taken years for villages to get a revamp, seeing most of the pre-1.7 complexes get updated to the less cubey, more decorative style of monuments, igloos, etc. would be nice. Mineshafts have done this sufficiently and now villages too, but both strongholds and nether-holds/ fortresses are very lacking with awfully barren rooms for their late-stage progression in the game. Even End Cities, which are much more interesting, can be barren at times.
It doesn't have to be free loot or obviously useful appliances like brewing stands. Just some decorations like paintings, banners, item frames, armour stands with half-finished armour, etc. would add more ambiance to many places that seem dead to the point of making Minecraft a psychological horror game, instead of a mix between adventuring game and sandbox.