This is a variation of a feature in Iron Maiden: Legacy of the Beast, and it might fit here in this game.
Essentially, once per day you could borrow and use one hero off of an Alliance members roster.
The borrowed hero could be used for one stage, one regular raid, or one Titan attack.
The borrowed hero could not be used for war or special events.
The borrowed hero returns after use to the original owners roster.
The original owner receives 1 gem for having their hero used (a minor reward for participating in this).
What this does is give some people the chance to play and experiment with heroes that they may never own/develop in a non-game breaking way. It may also help some developing players to get over some trouble levels if they can get help.
Once the feature is used, it can’t be used again for 24 hours.