In an effort to curb some of the challenges presented by vaulting, particularly, the obstruction newer players now have in acquiring required characters, I've come up with a fairly simple means to ensure that almost all of the 3* characters become available at some point.
Replace some of the 2* champion rewards with covers from other, still related, characters. This will also add more value to the overwhelming number of 2* covers encountered in both vaults and tokens, despite 2* play being long since left behind by many of the most experienced players.
Each 2* would feed 3 3* characters, still granting one of each color cover (9 total cover rewards per champion)Some brainstorming on who would feed which covers:
Ares: Ragnarok, Sentry, Thanos
Black Widow: GSBW, Scarlet Witch, She Hulk
Bullseye: Bullseye, Daredevil, The Hood
Captain America: Steve Rogers, Sam Wilson, Black Panther
Captain Marvel: Captain Marvel, Kamala Khan, Gamora
Daken: Daken, Blade, Punisher
Hawkeye: IM40, Hawkguy, Vision
Human Torch: Human Torch, Dr. Doom, Dr. Strange
Magneto: Magneto, Mystique, Quicksilver
Moonstone: The Hulk, (Someone Else, Someone Else)
Spider-man: Spider-man, Dr. Octopus, Deadpool
Storm: Storm, Beast, Psylocke
Thor: Thor, Loki, (Someone Else)
Wolverine: Patch, Cyclops, Colossus
The 3* not accounted for here are Iron Fist, Luke Cage, Rocket & Groot, and Squirrel Girl; all of which are currently in tokens.