This is exactly what I was thinking. I think you should lose the toon, and it should probably be expensive to do as well.
That said, I was more interested in the idea before I had a bunch of fully sharded toons. It really hasn't taken that long, between trooper badges and iron coins, to fully shard all my main offense and defense toons, with the exception of Eddies.
Though with THAT said, the next nerf bat or other adjustment, new toons, or newly discoverd hidden gem toons or new strategies, I may again want an easy way to move shards from a benched toon.