Currently there are 14 monster dice (15 if the Colosseum die is added and 16 with the mimic die). I think one small problem with the difference in size is the fact that you can essentially feel for the bigger die. I don't think many people will play this way, but it is something to consider. With the Epic Monster dice, I would suggest an expansion that adds like 5 general monsters and 5 epic monsters. This would pad out the monster bag, decreasing the probability of people grabbing an epic and dropping it based on the feel (or feeling around for an Epic)
As for Epic monster's rewards, what if you gave some of the playtestesters what they wanted regarding Class experience. Maybe Epic Monsters reward class experience, drawing power from the epic corpse lying on the table, giving everyone the use of a new face of their class die.
Only problem with these ideas is that new monsters might require new spells to draw from. Additionally, advanced classes would then require the epic monsters expansion.