I have really conflicting thoughts on REBELS.
On one hand, I think it's the best SW that's been created since ROTJ. It recreates the family atmosphere, danger, and sense of adventure of the OT, which was sorely missing in the PT and TCW series. I very much enjoyed season 1.
But on the other hand, I fear they're going to water down characters from the OT that shouldn't be watered down because they have nothing better to create -- a concern that preview only reinforced. Vader's shouldn't fight a middling Jedi and a child and the result being their escape. He shouldn't struggle to snatch away an object from the hand of a poorly trained apprentice. Vader, if he ever encountered any of that crew, should squash them like bugs with little effort. And ESB isn't relevant; he allowed Luke to play around with him because he was his son. One of the sins of the PT is that Anakin almost never actually did anything that was intimidating to the point that would warrant him becoming the fearsome Darth VADER.
Darth Vader shouldn't be a villain of the week that's continually defeated. In the TCW, they ruined (or, more appropriately, piled on the post-ROTS wastage of) General Greivous precisely this way. Prop up Greivous for ratings, embarrass his character again, and let the good guys get away. The result was that his character became a lame joke.
I hope they use Vader sparingly. And if not, he had better extinguish important characters along the way. Regularly.