mumboi makes a good point. Mooks will probably decrease in numbers as time wears on. However, I foresee multiple solutions to this problem:

Solution 1: Don''t give a rat''s ass. Fine, there aren''t any more hordes of mooks. Don''t make the game depend on them. Have passable AI, some decent challenges, and plenty to do. Mooks aren''t that big a deal to players, who just kill them.

Solution 2: Make mooking tempting even to paying customers. If you go with the "possession" theme, let paying customers "inhabit" monsters and NPCs that demo-players couldn''t, or else let them level up the beasties. Heck, you could even let player''s save beasts as a character file, thus leading to...

Solution 3: Make everyone a mook. "Mook" becomes an inappropriate term at this point, but let players play as any monster. The genuine mooks, who don''t pay, can only be weak, introductory level monsters, but once your money''s in, you can level up, access more impressive "hosts", and generally get more out of the game. I hate to suggest monster transformation and growth as a game element, since it''s so damnably similar to certain Japanese fads, but it''s a possibility that may be worth investigation.