This would actually also be a great way to increase iterations on abilities/bugs as well, 10x more testing per game and all. Naturally that's not exactly the case but still.
One problem though, for a lot of champs they need special interactions and coding for one for all mechanics. Like Syndra for example, either coding to allow for control of other spheres or a a way to differentiate your spheres from others. Naturally this will be more or less difficult based on the champion's kit, and could be used later when one for all comes back for live, it's just the matter of "is it worth the extra time?". It very well could be, I don't know.