I like blade's idea a lot, and I have an idea on how to implement it. The x strongest monsters in the game (Barring the 3 bosses of course) have their weights divided in to 2 parts (One far smaller than the other). The larger part is given to the greater monster, and the smaller part is given to a horde of level 2-4 monsters.
Let's say you gave 4 weight to arach/mancubus/AVs and 1 to their respective hordes, on level 20 of hard which gives a current challenge of 104 according to the danger level thread, you might get 3 arachs (27), 1 mancubus (12), 4 cacos (24), 2 AVs (28), 2 imps/a demon/a fh (Arachno horde, 9) and 3 lost souls/a captain/a sergeant (AV horde, 14)
All you need to do to make that a nice, balanced level is make sure the hordes don't split up after they've spawned. Perhaps make them all unable to open doors or something so they're at least in the same room.
To me, this doesn't sound like it's giving away free kills either, because imps, captains, lost souls and demons can do major damage if they're given the chance to hit you (There's 3 or more, you're gonna have to ignore one), especially if they happen to spawn near a greater monster (At which point I'd probly want to bring out a BFG or run away)