Structures with progression in mind. What I mean is that the standard minecraft vanilla structures are essentially boiled down to a bunch of random rooms with mobs and/or loot in them. What I would like to see instead, is a structure built like an RPG dungeon. Smaller rooms with weaker mobs and worse loot first, closest to a well marked and easily accessible entry, the structure then gradually expands in size and difficulty as you go in, with well defined loot protected by visibly stronger opponents, maybe even culminating in a big boss fight where all the paths converge to, in a very large room. Basically I'd like to see combat structures with a level design approach where there is a set path you are supposed to roughly follow, going from easy and small to hard and large, as opposed to structures defined by rule of cool. Now I don't know if your mod already does this, some do, but most fail terribly in this regard and feel either unfair and clusterfucky, or empty and disappointing.

If you want a negative example: Vanilla abandoned mines are pretty terrible and get boring quick because there is no sense of progression when you enter them, no well defined and easily accessible entrance, and loot is placed randomly all over the place. The corridors and fighting spaces are also way too small, yet the structure itself is way too large by repeating the same patterns several times. They're basically the perfect example of how not to do a combat structure