Pikkon is actually the only decent choice I can think of. Cooler is just derivative and the shadow dragons need to be fixed pretty bad. Boojack had a nice look but he did not bring up anything special.
Janemba and Hildergarn both look fun and had cool moves. I don't necessarily see them as fully fledged characters but you could redo those ideas and make them better.
Shadow Dragons wouldn't be the same, imo, because you couldn't do Nouva Shenron being Goku and Grandpa Gohan's Ball. He's have to be his own, unrelated thing, instead of the emotional ties to the protagonist.
Bring on Cooler, finally. With him back in the drivers seat of an empire, it's the perfect time to introduce a rival.
Cooler doesnt fit in current continuity. It wouldn't work. Uub is a good choice. I think a super Namek is needed right now. An evil, extremely powerful Super Namek. Cant think of a good one from a movie or anything though.
I always wanted them to follow up on Piccolo being trapped in Hell in GT. I thought a good story would be he gets overwhelmed by all the baddies so he is forced to fuse with his original self, Demon King Piccolo. With his new power he starts dominating Hell but the evil starts to overtake him and he craves more power. He fuses with all the dead Super Nameks like Slug and then escapes Hell. Once back in the real world he begins his rein of terror. The interesting part of this is that all his friends would be hesitant to hurt him while he would be trying to kill them.
Gender: Male Location: Some abandoned space station
They could just say/reveal that Cold didnt tell Frieza about having a brother because he didnt want the two to compete at that time or something. Having a rival like Cooler would give Frieza a new purpose in his conquest for U7.
And I mean detractors can say otherwise, but Cooler wouldnt be as much a staple as he is in games and other media like Broly if he wasnt popular. He would no doubt be the next in line if they were to canonize another villain imo. Getting his what-if Golden form in Heroes alone is more love than any of the other movie characters have gotten in recent times.