I think it would be a neat idea to have a galactic war type challenge set up.
Something like :
100 or 50 crystal entry every day
All characters useable
Characters defeated stay unusable until a set refresh point. ( like 20 to 30 battles)
Opponents all top tier but health and abilities refresh every 5 battles
Cash rewards for each win doubling every 4 battles
At every 10 a chrome character shard issued changing each week
Would have to have a max battle level, maybe 60 battles
Could also be used for unlocking new characters by finishing or say 10 shards of that weeks character.
Would be challenging
Would make you fill your roster
Would make $$$
0