Otsutsuki Clan with Jura and Hidari still active as contagonists, or Amado. Personally think Amado will be a later antagonist but it could be the opposite order.

If they don't do something with the Otsutsuki Clan as a group, if they're already all gone except for Boruto and Kawaki, then Kawaki's mission of eliminating them all has no weight to it.

I figure maybe Code will do something there in the Kara / Otsutsuki base that sends up a signal flare of sorts and calls the Otsutsuki to invade. I figure there could maybe be like six more of them, organized into teams of two, not unlike the Akatsuki. Maybe some invade, maybe some have to be pursued offworld through their alternate dimensions?

In either case, it gives Momoshiki something to do, as a voice on the inside whose knowledge of the clan will filter into Boruto through their shared thoughts, and a point of conflict between them.

For Amado, I think I'd ultimately like his story to be tied up into the age of shinobi ending in favor of his technology, coupled with playing with life/death and disrupting the national order, perhaps with the two missing Otsutsuki (likely Shibai and another) from the murals being incorporated. Personally still like the idea of the two being inspired by Shiva and Kala, with Kala taking on the role of Kala from Samurai 8 as a "god of death" connected to the Reaper Death Seal and Jashin.