I actually found the Titan Master gimmick less intrusive from a design perspective than the Combiner Wars gimmick. A lot of the Combiner Wars figures felt incredibly generic and overly simple because of the fact that they all needed to be a robot, vehicle, arm and leg, and the limbs had to be the same size across the line so they could be swapped around.
The Aerialbots were a wash because of this - three of the limbs were essentially the same figure (all four, if you're counting the UW release). Same goes with the Stunticons. I will concede that the Protectobots and Combaticons were at least insulated from this because of the variety of alt modes making up the team.
I'm with the rest of the posters in the thread who prefer lines without overarching gimmicks. If there is to be some unifying theme let it be something like Animated (dynamic head reveals for the transformations, and a line wide theme of Autobots using melee weapons only), or Studio Series with it's line wide consistent scale.