They've painted themselves into a corner with this tier system. For long term success they would've been better off making every character "viable" (by that I mean star levels do not=power level), but the star system being cover rarity only. Had they thought of that from the start they could've more evenly distributed the characters among the rarity tiers and simply added more as needed/wanted without hassle.
For instance Marvel Now Magneto would be a 2* rarity character still because it's the lesser known version of the character. Classic Magneto would be 5* version and much more rare to acquire, yet neither would be more "powerful" than the other. People would still chase 5* Magneto like crazy because of his popularity alone, but add in a fairly unique power set and they'd be just as driven as they are now.
1* Mac Gargon Venom-Not well known version so people won't chase
3* Flash Thompson Venom-Current Venom, not as popular as Eddie Brock but has a fan base
5* Eddie Brock Venom-Most popular version and people will chase, absolutely
Not every character would need 5 versions though. Multiple man, 2* character only. Mole man, 1*.
Again, * only denotes rarity (popularity, really), not usefulness.
Anyway that's how I would've built the tier system.
For instance Marvel Now Magneto would be a 2* rarity character still because it's the lesser known version of the character. Classic Magneto would be 5* version and much more rare to acquire, yet neither would be more "powerful" than the other.
I suppose it depends on your definition of decent. From the current ones that we've got, I would use these characters:
decent Yellow and Purple active : Squirrel Girl
Black and Blue active abilities : Black Panther, Mystique or Psylocke
red and pink active: Deadpool or GSBW
They're not the best, but they have their uses.
I agree a Couple more 3stars like gambit and rogue would b nice but devs won't make them now as there into 5stars and the super whales ain't gonna pay £1000 a week for a new 3star