gameplay wise kid Goku is fine, it's just I don't want another Goku since we already got SSB slots. I'm a little perplexed that they didn't give Vegeta/ Goku the SSB transformation as a Level 7 super.. I mean freeza does the same. That way you can add kid Goku and the roster feels fresh.
All the transformations should cost super bars, with SSJ3 having a short timer (even if it's just for aesthetics)
I'm going with the team is trying to represent each era properly. What we currently have in the game is the DBZ era, with hopefully Kid Goku or Roshi representing the DB era. Then SSB Goku and Vegeta representing the DBS era since the previews we've seen of them show them doing stuff that's only been in the DBS tv series (note that Golden Freeza and Beerus debuted in DBZ movies technically).
That or we're getting a Super DLC pack, with SSB Goku and Vegeta being the first part of that.