Boat transformers. I know we're getting a new one in the form of a RotF Scout toy, but boats and things are still really underused.

Also, a gimmick that I'd like to see not so much return as just appear: Boat transformers that FLOAT. Since it kind of defeats the purpose of having a boat alt-mode if it cannot float in water.

"What! The Decepticons are attacking! I'll go and help stop them! Seaspray, transform! Wait, why am I sink-*glub glub glub glub*"