What about off world autobots such as the junkions? A few of those with optimus at command could free some random sparks (I'd leave rattrap, LOL). Or alternatively you could bring back a fan favourite in (wait for it) Waspinator. Having been left on earth he could have stayed hidden with a covenant of primus (perhaps stolen from the heavily damaged nemesis) to read for millions of years and possibly begin to understand, making him a sort of new Alpha Trion (like from the WFC exodus book). He could return to cybertron at just the right moment having knowledge of what he needed to do from the covenant and having perfected his own transformations from studying the book. These are only ideas I'm rolling off the top of my head remember so probably flawed by other series events.