My favourite gimmick was the Unicron trilogy minicons. Some of the minicons themselves were very bland or uninspired but the idea of having smaller bots power up or unlock weapons and modes were good fun, plus the extra bot was always good. The Cyber keys to do the same thing were not quite the same but some of the best gimmicks were the unlocks they provided. I think along the same lines as Tazzerath, the extra small bots provide lots of extra fun, ala micromasters.
The auto-morph technology that came with some of the movie lines were a great idea, again not always executed well...
Looking For: Wreckers Saga TPB Collection (with Requiem)