How about introducing Thronebreaker-crystals?
I know it has been talked about several times. But at the state of the game in my opinion it's time to rethink about an update for the Cavalier-crystals and give the higher player the possibility to purchase Thronebreaker-crystals.
As a Thronebreaker with 100 or more 6* you can invest 175 Units to have the summarized probability of 3% to get another 6* but 50% for a complete useless 3*. That's not attractive. Only for someone how just started the game maybe.
This would be a good output rate for a Thronebreaker-crystal:
40% - 4*
10% - Legendary (4* or 5*)
30% - 5*
12% - Mythic (5* or 6*)
5% - 6*
3% - 6* Nexus
Even if it's around 300 units to purchase, it would be attractive.