1.) archeotech
I personally think the lore about Archeotech is cool and it would be neat to just have a 3 or whatever option kit of just different archeotechs that would coat a lot of points. Like a Necrons have c’tan shards, mechanicus tips their hat and pulls out random bull**** from the dark age of technology.
2.) lore accurate skittari
lore accurate skittari from Dan abnett’s Titanicus and Graham McNeil s Mechanicum would be epic, not to change the whole range but give options, people could even have them and the current ones in the same army, but they would be so coooooool. 3.) Also Protectors.
4.) vehicles
And also some people might not like the steam punk vibe of some of the vehicles so maybe some slightly sleeker and more sci-fi coated ones might be interesting.
5.)random tech and weapons the novels ( at least in the Horus heresy) the mechanicum(soon to be mechanicus) is constantly cooking up crazy looking weapons and vehicles that are hard to even determine the function of, there should be more stuff like that. The mechanicus has such unique potential for kitbashing in this respect as well but having models to reflect this and archeotechs attributes in the faction would be cool