This is a problematic statement. There are tons of definitions of viable out there. SOTDs generate interest in the game and whenver you wait too long because a save isn't "viable" or "perfect" means that you're missing the point. SOTDs are just that, an interesting setup to try out that is different than what may be commonly played thus far. It's not intended to be perfectly balanced nor perfectly competitive. That should not be an excuse for not selecting a new SOTD.
I doubt anyone here has applied serious scientific rigor with regards to what makes saves balanced. Just choose a save and get on with it and have fun. There are plenty of suggestions from multiple users to go on.
TLDR: Quit stalling and select a new save of the day, it won't be perfectly balanced; that's ok!