Suggestions for a way to keep players happy
Why not let players enjoy a weekly or monthly event where we can catch a "LIMITED-EDITION" monster that currently exist already?
For example, you can have Draconius available for catching in normal mode just like devil and angelo but have a Blue Draconius instead of the regular one. Red Rhor instead of the regular rhor, etc. Color-swap of the already existing monster will not require much labor.
If possible, maybe add a blue draconius as a water type instead of fire? Have the stats and talents to be exactly the same, but just have him as a water type.
This would give more variability into the game without compromising the balance issue since these are just epic monsters.
This will keep players more engaged into the game because of the nature of "limited-edition." By doing so, you're rewarding players who are actively playing the game.
A lot of phone-based rpg games do this, and this is one of the major way that game companies like Mobage utilize to keep players happy.