In regards to Punch Out
-I personally wouldn't do King Hippo to be honest. to be personally honest, King K.Rool kind of fits some of the Archtype of King Hippo, Especially with the focus on the Belly (where both had their belly sweetspot be a Weakness, and also a threat to conquer in itself if you take Title Defense King Hippo into Account
I think whoever would be the Boss character of Punch out should be big, but also not very Laggy, So I would go with Bald Bull or Mr. Sandman
-If you wanted to make a Boss character from Punch Out playable and not just a boss, How I'd balance them is that they would have better stats than Little Mac on average: Around the same speed, Much more Stronger, Heavier, Perhaps better Aerial Game, More range, etc
Their Weakness would be that if you hit them while they are in Attack animations, they'd be much more vulnerable, either by higher knockback or Higher damage. This encourages interaction on Both sides, and also represents on how you typically defeat the bosses of Punch out
-If you wanted to have Doc Louis play a bigger role in Little Mac's Moveset, perhaps make his taunts have a percent chance of Doc Louis throwing a Chocolate bar to give to Mac.
Though to be honest, I think they did make Little Mac's Moveset to represent the boss qualities of Punch Out. this is where the Super Armor and the High damage of Mac's attacks come from. Mac doesn't do as much damage per hit in his home series as he does in Smash, unless he has KO Punch or Star Punch. Honestly in that case, I doubt Boss character from Punch Out. If anything, they'd just use Giga Mac since there's already a model made for that and such