In future there are going to be more fixed structures, like Minas Tirith or the already existing Lonely Mountain. Historically, it's a 3 day journey from west to east entrance of khazad-dum, which of course would be too big in my opinion to make it a fixed structure. So my idea is to make it a biome like the rest of the misty mountains, only there would be a random generation of halls and tunnels beneath those mountains that would all be at the same y level. Although there should be a few fixed structures, like east and west gate, tower of durin or even some greatest halls.