I think Trailbreaker would make an excellent Prime. He's good natured, he's good in a fight, he has a goddamn forcefield! If you look in the comics then you'll see that he can take a beating and keep coming, if you watch the cartoon you can see that he's useful as a warrior and a defender, all we'd really need to see from him is the "strategist" part of his "Defensive Strategist" function. He's a really under-used character and that's always bothered me because he's got plenty of potential.
His secretly depressed "I'm a drain on resources" attitude would be replaced with a cheerful confidence and his force field would go from a static wall/dome/sphere, to one he could shape Green Lantern style! His new moniker would be Champion Prime!
Also: This would be his vehicle mode!