Rather than sell covers once a character is fully championed why not have the rewards loop? The rewards reset and players can add covers (without gaining levels) to obtain further iso, covers, tokens, and HP.
The difficulty in obtaining the various level of covers, 2-5*, correspond to the rewards anyway, so there will be no significant boosts to players rosters. What it would introduce, and what is very much needed right now, is a small boost to iso levels.
Even if players had to pay the Champion fee again it would still be better than future covers going to waste
My ideas:
1st- After reaching maximum level, you will be able to sell the hero for 100% of ISO8 you invested in his upgrades+promotion cost (plus HPs you are getting now for selling upgraded champions).
With this you can sell hero for tons of ISO you can use elsewhere OR you can simply get 13 cards, max. him again (for ISO8 you got from selling), promote again and start getting rewards from champion levels.
2nd- https://forums.505go.com/discussion/38289
This idea of mine could (among other things) greatly improve the benefits of having maxed champion covers and multiple of the same covers. Imagine the army of 2* covers that is earning you the ISO8 while you are offline 
It would be great to reset champion rewards. Right now, I actually sold a few of my max champed 2s so that I can do it all again. Now I kind of regret it, because ddq is much harder, and this means more 2s to champ before I can champ the 4s. But anyway I guess we’ll just see. I’ll probably end up champing 2s and 4*s concurrently, I guess. I’ll collect all my iso and see what happens.