So when this series premiered I went back and I watched the Rebels season finale that featured Vader and based on the words Maul used I knew he had to meet up with and lose t Vader at some point. I thought it would be at or near the series finale.
Obviously we got him much sooner.
I still think Vader's return is a possibility. So while I don't necessarily want Maul and Vader meeting every season, I don't think we've seen the last of him in this show.
Maul's goal is to take down Sidious. I wouldn't be surprised if Maul vs Vader plays out similarly to Luke vs Vader in Empire Strikes Back. Fight. Retreat. Fight. Retreat. I expect Maul + Devon will at some point have a slight edge against Vader. In that moment Vader might have something to say.