Stop asking for all-plastic 20th Prime. It's a disaster waiting to happen. The toy structurally needs the metal, or his waist would snap like a twig.
To be honest I'm not sure what they could really do for the 25th anniverssary. Prime is kind of the leading TF icon and he's already been done, and MP Megs can't be released in the states, so that's a no go. I think having Screamer as the 25th anniversary figure would work. Not quite as impressive as Prime, but in terms of what he know from Hasbro I'd say this is our best guess so far. QFT!
Probably a re-issue of the original Prime for 25th Anniversary. And no, I don't know that for fact, just assuming. Shoot, maybe they'll do a complete G1 re-issue release to a major retailer. Include IDW Mini-Comics for the character or something. Do something like... Optimus Prime Jazz Mini-Bots (Bumblebee, Cliffjumper, Brawn, Windcharger, ie the keychains) Starscream Soundwave