I would love such a feature added eventually when all the current bugs are ironed out as much as possible.
Often upgrading new squads and just wanting to try out different teams and how useful they would be even at a X amount per day and crystal refresh cost like in the arena would be good.
You are right, GW is no longer an option because you one hit kill every one you are put against all the way until the last battle and even thats easy, but before being able to sim i was so glad it was this easy :D