Nooooo. Bad idea. This would mean that those who train would have to train a lot, and those who only train a couple of dragons (eg newbies without a proper trainer, people with little time, those who can't train much because of disabilities) would be discouraged from training. Go hard or go home. It'd be so toxic.

The new optimum strategy would be to discourage anyone but a few to go into the coliseum and funnel all the levels to them.

Dom as it is now is far from perfect, but it's also really hard to improve on.