Restful Respite - a delay floor, inspired by mid-dungeon save points you see in a lot of RPGs. No minions spawn here, nor do patrols go through it or hunters track down heroes there. Single or small groups of heroes that manage to reach the Respite will wait there until either A) a certain amount of time has passed or B) a full party of four can be assembled before venturing further into the tomb. Possibly with a minimum wait time so that if a full party comes across it they won't just bypass it entirely. It gives you a bit of a breather to get your defenses set up but at the cost of also giving the heroes the opportunity to organize.
Blasphemous Basilica - Cultists are nice and all, but a proper dark lord really needs a more organized religious service devoted to himself. Dark priests to lead the masses and drive them into a frenzy! That sort of thing. I can see this floor offering upgrades to both the Dark Elf and the Heretic paths. There are a couple archetypes of priests which could come out of here: a typical fantasy healing priest, a fiery orator type who gives buffs to other minions he is in an encounter with, a cult leader who sacrifices the minions he's fighting alongside (does damage, adds fatigue, I dunno) to buff his own stats. There are many options!
Ancient Aquifer - Who likes water elementals? Tyrants like water elementals! You can never have too many element-themed units, and an underground cavern with a great lake inside of it is a great place to recruit them. Alternately, if you've been going down the route which gets you a gross sewer full of slimes perhaps you wind up with an Adulterated Aquifer instead, with sewage- and industrial-corrupted sludge monsters. Polluting for fun and profit!
Conqueror's Crypt - A floor down the zombie line which produces more powerful, well-armed zombies. Or maybe ghosts. I dunno. Perhaps it gives you a guaranteed champion undead unit. The late hour is starting to get to me, I just wanted something other than "creepy" to go with "crypt".
Licentious Laboratory - Got some Crumbling Catacombs? Got a Wicked Workshop? Put the two together and play god by creating crimes against nature Doctor Frankenstein would be horrified to witness. If you can think of a better "L" word than "Licentious" feel free to swap it out, it means immoral but the more common definition is a sexual one and that takes it to a weird place.