Before we get into characters, I'd like to see a full revamp of the game's core mechanics when we get a sequel. Guilty Gear STRIVE is by all accounts a huge departure from the Xrd subseries, so ArcSys are no stranger to majorly mixing things up with their fighting game sequels.
I've been pondering the idea of making an entirely OG Dragon Ball themed sequel (or prequel I guess) as I've been rereading the manga from the beginning. To make it, they'd have to ditch the super-dash mechanic entirely as most characters prior to Z can't fly. I think this mechanic is what's stopping iconic characters like Muten Roshi and Mr. Satan from being included as DLC to the current FighterZ. The only way Roshi could fly is if he had Baby Gamera as a mount, which would look hella awkward. However, high-flying action is what Dragon Ball is all about, though there's plenty of other ways they can recreate the kinds of epic battling without these mechanics.
In terms of characters, I've had a few ideas, but really I'm quite happy with the current roster. Not much more is even needed, as OP said. I'd like to see certain characters like Krillin and Piccolo revamped with more moves introduced in Super, like Krillin's 100x Taiyoken.
Roshi - Yeah, it goes without saying. His moveset can incorporate his unique techniques like his Thunder Shock attack, the Mafuba, and as his Level 3 he destroys the Moon with a MAX Kamehameha, sending shards of rock down on the opponent.
Imperfect Cell - I had some good ideas for how his moveset could be distinct from Perfect Cell. Of course, he'd incorporate his tail into a lot of attacks, and as a Level 3 he'd simply drain his opponent of all life force. He could also use the Makankosappo in midair.
Karin - A weird one, to be sure, but hey, I just finished rereading the RRA arc. The game needs more weird, wacky joke characters, dammit! Karin would be really challenging character to adapt, but he's got some sweet cane moves, the Zanzoken and I've heard that in some games he can summon lightning bolts from the heavens. As his Level 3, he forcefeeds the opponent the Ultra Divine Water.
Android 19 and 20 - I imagine them as a true tag-team character. You could swap out between 19 and 20 at will (the one you're currently not playing as will always be hovering in the background). Their fighting style of course revolves around absorbing energy.
Cabba - Yeah, there's not much Cabba can do that Vegeta can't, but he has a vertical Final Flash?
Tapion - Although I love Tapion, I'm not sure what they could really do to differentiate him from Trunks. Maybe he has a unique mechanic where he's constantly fighting off Hildegarn's influence, then eventually he unleashes him in his Level 3?
It was an offbeat idea that would never happen, but I was daydreamed a concept for a completely relaxed Farmer Goku who fights more like a gag character, throwing tractors and radishes at people. Going back to his roots, in a way. There's already way too many Gokus though.
I am 999 years old. I don't give a f--