Now this is merely in the case that some characters don't get new molds (or CGI models) made out of them and are simply redecos of past figures. For the Optimus Prime mold, I think you could easily make an Ultra Magnus or Motormaster out of him. I think you could retool Bumblebee's mold to resemble Jazz and give him a new paint job. Arcee's figure could be done up to look like Chromia and Elita-One (as well as other Female Autobots). And Skyquake's figure could be made to look like any of the Arielbots (including non-combiners such as Jetfire and Breakaway) with a simple head retool.