Challenge System.
Why is there no sort of challenge system? All the top games have them. Daily, weekly, monthly challenges and a big categorised list of challenges with no time limit would be nice.
Daily challenge: defeat 10 6* champions, beat a champion 3 tiers above you (1* beat a 4* etc). Achieve a 100 hit combo, 200, 300 whatever. Defeat 5-10 enemies in alliance quest/alliance war.
Main challenges with no time limit: play X amount of quests, defeat X amount of bosses/minibosses. Defeat 1000 enemies, knockout a champion without landing a hit, complete RoL with no revives, defeat X amount of enemies in AQ/AW.
Character challenges: defeat [villain] with [hero]. Make the comic story relevant, defeat Magneto with an X-Man, Venom/Spidey, Mordo/Strange.
Rewards could range from low level stuff like potions to basic 4*/5* heroes, t4cc, awakening gems for the much longer or more difficult challenges. This would also give people a much better reason to use a variety of characters. (Defeat 2 enemies in AQ/AW with [insert average champion])
A challenge system would really improve the game and put seldom used champions to good use. Rather than buff/nerf, give people a reason to use a variety of champions and challenge themselves.