Given that consistency and coherence have been largely irrelevant to the franchise so far, it's hard to see why a reboot would be neccessary.
Just take whatever is working, make another movie and call it "TF5: Age of Galvatron". No need to throw the baby-bot out with the cyber-bathwater. There's good stuff here to build on. In fact, it would be nice to see them develop things rather than scrapping everything and starting over. How many characters and TF models have they gone to the effort of creating only to have them stand around in the background or do one scene before they're killed-off or never seen again? Jolt, Que, Mirage... Jazz? Even Ratchet didn't really get a decent spotlight until his demise in TF4. Megatron looked so cool in DoTM so it felt like he was under-utilized with almost all of his scenes just being him standing around being grumpy before getting his spine immediately ripped out as soon as he decides to get involved in the action. Shockwave? Now that was an anti-climax. We get a cool two-headed version of him in TF4 apparently only to further taunt us.