As others have pointed out exhaustively, TCW!Grievous really is no different from ROTS!Grievous. If anything, TCW!Grievous is preferable-- the general has held his own in combat against Obi-Wan, Kit Fisto, and Ventress at various points, whereas Obi-Wan casually manhandles him in ROTS up until the contest becomes a hand-to-hand one. Likewise, Filoni has reminded George a time or two that Grievous must be seen as a credible threat even if he can't take on the most powerful Force users in the galaxy. "Massacre" was a step in the right direction. Like others, I personally think that Grievous should slay a handful of "redshirt" Jedi throughout the series (if not all at once) to justify the general's confidence in taking on trained Jedi; if he kept losing to them in single combat, then he wouldn't be eager to duel Obi-Wan alone in ROTS-- no matter how arrogant he is. The most important thing Filoni and co. need to do is spend time portraying Grievous as a threat in naval combat; we need to see him win fleet battles, even if they're against no-name commanders. I reiterate a point brilliantly made by @TaradosGon : if Grievous isn't a military threat, then it doesn't make sense for the Senate to vote Palpatine further powers. Why people don't respectfully remind Filoni of this point at Celebration VI or things like it eludes me.