Think mythics are just going to be more of the same as far as power creep is concerned. Part of the issue with power creep is that you only have a handful of viable monsters at the top at any given time....lack of diversity and strategy in using them.
A different approach might be to take the older monsters that are already out there, and give players a way to earn extra traits on those, for example. Could also create a way for players to earn additional stronger attacks on those as well.
Basically, give players ability to play to keep more monsters viable for a longer time, thereby creating a likely more creative and diverse game play at any given time.