Most important of all, give the game at least two years of development. That way we don't just get stuff that should have been in the previous game in the first place.
First, they should have more character creation options. Xenoverse 2's is good, but that's how it should have been since the first game.
Add one more race, Androids, and give female options to Namekians and Freeza's race too, since there is no reason not to.
Have a class system for each race:
[spoiler]Saiyans would have Low-Class (levels up faster and turns into Oozaru and Super Saiyan 4), Elite (stats grow more with each level, can use Super Saiyan 1-3), Prodigy (balanced stats and growth, best transformations Super Saiyan God and Blue).
Namekians would have Dragon Type (levels up faster and can turn young), Warrior Type (better growth, can use Super Namekian), Demon Clan (balanced, turns into Giant form).
Majins would have Pure Good (the fat one, levels up faster, can get angry), Pure Evil (the skinny one, better growth, turns into a Super Buu-type), Pure (kid type, balanced, turns into Buff form).
Humans would have Turtle School type (level up fast, can turn Buff), Crane School type (better growth, can use the 4-arm technique), Animal type (balanced, can turn into a Man-Wolf).
Freeza's race would have Emperor (levels up fast, has Freeza/Frost's forms), Mecha (better stats, can turn giant like Meta Cooler), Prodigy (balanced, can turn into the Golden form).
Androids would have Infinite Ki-type (levels up fast, can turn into a Super Android), Absorbtion-type (better growth, can use a Giant Robot like Dr Wheelo or the Battle Jacket), Biological type (balanced, has Cell's transformations).[/spoiler]
Of course, add more characters and stages.
Give existingcharacters their missing forms and have all the movie and OVA villains, in all their forms (even if some are boss-exclusive like Dr Wheelo, Fat Janemba, Meta Cooler core; though preferably they can be playable but you can't use the other two team mates). Dragon Ball characters: at least kid Goku, Piccolo Daimao, Tao Pai Pai and Muten Roshi. From GT Baby, regular Baby Vegeta and Ledgic. Everybody new from Super (though I guess we'll get the missing ones up to this point trough DLC)
Have more clothes slots, so you can combine different outfits, so you can wear sunglasses with a mask, or Saiyan armor over other clothes (not just Vegeta's or bare skin) etc.
Storywise, allow the player to either be a Time Patroller or a Time Breaker. Towa and Mira replace Kaioshin in Trunks in the hub city (with the city also getting a little reskin) and roles are reversed, with you having to defeat the heroes.