Gungan, the story of the secret Sith lord, DARTH BINKS.

They call it all conspiracy theories, but brothers, I know the truth.

Not really all about future sith but Umbarans have always had a soft spot for me so I’d love to see them in a future antagonist role.

Trandoshan

I think the species of Kalo'ne would be absolutely terrifying if you gave them red eyes! I mean, if you look up that page, that alien is supposed to be female! What do the males look like!? An even worse possibility is the Givin... eeeh....

There have already been Umbaran and Duros Sith Lords: Darth Ruin (Legends) and Darth Caldoth (Canon) respectively.

^So? How many human Sith Lords have we seen?

^I'm not saying there can't be more, I'm just saying you don't need "future Star Wars media" to see them like OP seemed to suggest. Maybe I misunderstood.

We already have a Gungan Sith Lord

Arconas definitely look the part.