STP = Nobody fits. Chester was pretty good at singing the same old set list these guys want to continue playing forever; and I think that was their last & best option. Trying again is just more beating a dead horse.
New Project = Chester would have been their best bet for a NEW project, with NEW music, and a NEW style. They all could have gone the Audioslave route and broken free of their past sounds/bands... but they blew it.
IMO, the DeLeos & Eric's best bet is to find a NEW voice, or at least a relatively unknown talent. This is why major movies often cast unknown actors in lead roles. New talent offers a fresh start, new perspective (voice), and it eliminates comparisons.