Would it be viable to add 4* cover rewards to 4* Champion tracks, so that the tier can help cover itself?
Using the current boosted characters as an example:
What if Thing gave you Mr Fantastic, Me Fantastic gave you Teen Jean, Teen Jean gave you Gwenpool, and Gwenpool gave you Thing?
Link them in small groups (excepting joke/trophy characters, just let them buddy each other) so there are other routes to covering characters.