I just don't want another case like MK from any of the characters in the roster
No one does: MK was the cancerous wart on the ass of competitive Brawl.
But I'm not even the least bit worried about Bowser being OP or even close to top tier.
Here's a video from a day 1 Brawl tournament:
https://www.youtube.com/watch?v=N3JTr9fi0Rk#t=324 It's an Ike ditto, yet we all know where Ike landed on the tier list.
Even in the Brawl days, I had a reasonable amount of success playing as Bowser over wifi.
Why? For the same reason these guys won this tournament: no one knew how to fight Bowser.
And in the case of this tournament, many of these players did know how to fight Bowser: BRAWL Bowser. You can see a number of times when the opponent would try to get in Bowser's face and, unlike in Brawl, it didn't work. At that point, they seemed horribly confused about what to do next. This was a beast they didn't know how to fight.
Once the game is out, players will figure out the Bowser strategy and everything will be fine. Even if Bowser was somehow top tier or close, it won't matter: he'll NEVER be as game-breaking as MK because MK was the perfect storm of speed, recovery, TONS of approaches with stupid priority and a disjointed hitbox twice the size of his hurtbox.
MK is proof that Sakurai couldn't balance his way out of a wet paper sack, but this is the Tekken team working on it this time around and they clearly at least understand that a heavy character's attacks KOing at 0% would be meaningless if that character will never get a hit off in high level play.