I see too idea to avoid a lvl cap:
***idea 1***
And why not an auto-regulation by monsters of their huge leader? Monsters can attack themselves and recover slowly so if a monster gets mad and kills every other monster (and newbie

), he will of course win exp but lose hp. When a monster is too huge for the place where he is, others monsters make an union to wipe the huge one.
Why it is realistic:
only \"intelligent\" monsters can win exp. If they are enough wise to understand from their win, they are enough wise to ally to kill another one.
And it can be funny if a player see a lvl 50 monster fighting 25 others lvl 5 monster then try to go away without being seen

Why it is fair:
Monsters will never grow to much. If you encounter a huge monster, that\'s just bad luck, or you should have a rogue in your team to see if there was a powerful monster before rushing.
***idea 2***
Monster can become old. When they are alive for a fixed time, their strength/tougness slowly decrease until they die.
Why it is realistic:
That\'s the way the life work. First you are weak then strong then weak then dead.
Why it is fair:
Monsters will never grow to much. If you encounter a huge monster, that\'s just bad luck, or you should have a rogue in your team to see if there was a powerful monster before rushing.
Maybe the two can be combined with the Nikech idea: monsters live in clan, the more powerful one is the boss, but when he gets old, elite members of the clan want its place so challenge it until he fails.
Or maybe the system can be different for each monster.
Some monster shouldn\'t have exp, other should become old, and others should live in clans