I mean Ahsoka and Maul's stories continue in Rebels as a follow up from Clone Wars. I'm sure you could watch Rebels without having seen Clone Wars, but if you're going to watch Rebels I would highly recommend Clone Wars first.
Clone Wars & Rebels spoilers
- Maul being alive. Him just showing up in Rebels if you last saw him die in Phantom Menace leaves you with a lot of questions.
- Ahsoka battling Vader. It helps to know who Ahsoka is. Vader with a broken helmet saying her name is heartbreaking.
- Obi Wan and Satine. Maul and Satine. While Rebels doesn't mention her the relationship they had is important backstory. the Twin Suns episode is the conclusion to when Obi Wan and Maul first met, the finale. You can skip the middle, but it adds a lot.