What about some monsters that are able to summon more monsters or some elite monster able to call your spawned monsters next to him ?
Creative solution, I'll give you that. Play around with it and see how it works out for you. In all honesty, though, I suspect this elite monster will just become the favoured target when clearing rooms.
You could make a monster that can teleport to any empty revealed space and then summon a beastman warparty for 8 threat. All you've really accomplished is to create random damage that hits the heroes no matter what they do, which isn't exactly going to maintain the tactical appeal of the game. You could make a new kind of trap card that dishes out an equivalent amount of damage and it would probably go over better.
I still think that the best way to accommodate more than 5 players would be the two competing hero teams idea I mentioned earlier. Two competing hero teams will both have access to the same degree of attack power (ie: treasure cards) and they will (hopefully) have equivalent hit points and armor ratios, which means they'll be equally hard to kill off. LoS coverage to cancel spawning is a non-issue since the heroes all come in through glyphs. (You could declare inactive glyphs to be the "red team glyphs" and active glyphs to be "white team glyphs" and then distribute them around the map if you don't want both teams coming in through the same glyphs.) Whatever monsters the OL can throw out will be roadbumps, as would seem inevitable with this many heroes, but that drawback would be lessened by the fact that the monsters are no longer the main attraction in the killing fields.