How to fix the act 6 5/6* only problem.
I’m going to say something controversial here but I agree with the fundamental idea behind the 5/6* only requirements in act 6, if you’ve only got good 4*s then why would you need 6* shards, T5B or T2A, three of the primary rewards of act 6?
However many situations in act 6 and some champions themselves require synergy partners (the Angela+Heimdall problem comes to mind here).
To keep both of these reasonable why not allow 2/3/4*s as synergy parteners, but once you join the quest they can’t be used in fights, or a node that instantly KOs any Champions under 5*. This means that these synergies are accessible but the champions can’t be used, so you still need 5*s to actually do the fights but not a huge, deep and niche roster.