The Return of Savanti is... well, it's really just a filler episode, but it immediately catches your attention by being a two-parter. When I read the episode's title, my first thought was: Who's Savanti? That's right, when I first saw this episode, I had completely forgotten about Savanti Romero. But how can you blame me? Before this, Savanti was just a minor villain who appeared in one episode, and was quickly revealed to be a mere pawn used by Ultimate Drako. In "Time Travails", Savanti didn't leave much of an impression, so I wasn't particularly impressed that the episode's title was announcing his return. Now, after doing some "research", I have discovered that Savanti, Renet and Lord Simultanious are all characters from the original Mirage comics, and this episode is a direct adaptation of one of their stories from those. I think it's great that this series actually uses material from the comics (a rarity among TMNT adaptations), but since Savanti had previously been a relatively easily defeated foe (not to mention a pawn for Ultimate Drako), I found it jarring to suddenly see him presented as a Badass Evil Sorcerer
who can control dinosaurs and threaten the entire timeline. It doesn't help that Savanti is a very one-note bad guy: Half of his dialogue is just rambling about how "The time sceptre will be mine!/The time sceptre is mine!" or laughing maniacally.
And another thing: Why did this episode need to be a two-parter? It's based on a very simple premise that could easily have been fit into one episode: Basically, it's about an old foe who seeks revenge/world domination and the turtles plus Renet go and stop him. The end. This story just barely manages to fill two episodes by having lots of Padding
in the form of the turtles fighting dinosaurs and escaping a dinosaur stampede. In the end, the turtles and Renet end up stranded in the past for three months, but then travel back to the present just after they left, making this seem entirely meaningless.
The Return of Savanti isn't a glaringly bad episode, and it just may have been saved because of the awesomely epic final battle against Savanti. But again, it is a filler episode in disguise. As far as filler episodes go, it's actually pretty good, but it clearly doesn't think of itself as such.