2* troops are the only troops that can be used in rare tier Challenge events, and Tournaments.
While epic tier Challenge events, and tournaments, allow 3* troops, they would also allow 2* mana troops. So deciding between 2* mana troops and 3* crit troops, for each hero would be part of the tactics and strategy.
Adding Mana bonus 2* troops would benefit all heroes with costumes and classes.
2* level 15 troop
Attack +10 Defense +10 Mana +15
2* Mana troops would increase the demand for emblems and costumes.
My preferred monetization.
Especially since new 1* troops have already been added to the game ( see notes )
Or 2* Mana troops could be a bonus 10% roll every time a troop summons is preformed.
Like an HotM summons but 10% chance instead of 1.3% chance.
Because of color stacking ( see notes ) troops have the most impact on rainbow teams.
Rainbow teams are excellent on defense.
Defense heroes in center, and flank, often benefit from crit troops because of the high +DEF an high +HP.
Defense heroes in flank, and corner, often benefit from mana troops because of the increased triggering of special skills and crit troops because they live longer to use their normal/ slash/ physical attacks ( double normal damage can be substantial with emblem heroes ).
Because of this, 3* / 4* crit troops and 4* mana troops are very useable at level 1 ( see notes ). In fact, all troops were original level 1 before leveling was added with the Barracks building ( see notes )
Healing is pretty meh.
If it was like talents “X% chance of 2x Healing” then it would be useful.
I would not mind if 3* troops had the healing bonus replaced with mana bonus.
Or a fourth 3* troop was added.
But adding a fourth 3* troop ( not F2P friendly ) or retroactively changing 3* healing bonus troops ( players who got rid of them ) both have problems, especially if you also add a 2* mana troop.
Rare/ Epic tier
I do not see any situation were just adding 2* mana troops to the game, like they did red 1* troops and yellow 1* troops, is not the least disruptive way to add mana troops for rare/ epic tier challenge events, and rare/ epic tournaments.