Ideas if Bowser had a 3D platformer.

If Bowser has a 3D Platformer, I think it should either be a game where Bowser tries to beat some other villains to prove he's the best, these other villains would be Wart, Captain Syrup, K.Rool, King Boo and maybe a couple more or a game about Bowser's origins and how he became so powerful
 
Definitely, yeah. Nice tie-into Super Mario Land as well. So it definitely could be like Tatanga, Wario, and Wart who kidnapped Bowser Jr as the game's plot. A good measure I think for various worlds to go through as Bowser would be to throw the Koopalings into needing to be rescued as well too.
 
I agree that, should this game exist, it should be conceived as vastly different from previous games. Nothing that already exists should be a comparandum. Bowser would need a specific gimnick on his own - breathing fire and destroying things with his size is too basic, as it has already been done previously (Super Paper Mario, Super Mario Odyssey). When other Mario characters took the spotlight (I'm mainly thinking of Wario, Yoshi and Princess Peach), they took the series into new directions. I think this should be the case with Bowser.
 
Maybe the gimmick could be similar to something like Dry Bowser, Fury Bowser, and Meowser in some ways maybe? That's at least my two cents.
 
Bowser is not particularly known for being very agile, so a platformer would in some way have to do something radically different to Bowser and his abilities in order to make platforming gameplay feasible. If you just make him jump and run just like that, he loses the weight he is known for and will feel weird, but if you make him slow and weighty he will feel bad and cumbersome in a platformer.

The best I can imagine would perhaps be a platformer where he can switch between various gadgets on the fly to enable jumping high, running, attacking and so on, but that also feels like it breaks up gameplay flow more than anything.

I think Bowser games would much rather have to find another genre. A Bowser-themed 4X strategy game would be funny as heck. Like Civ, but with Bullet Bills, Goombas, Koopas...
 
How about Bowser's Middle Kingdom, where Bowser goes on a big medieval adventure after falling down an ancient well? Zelda-style baddies would be present, and the goal is to progress through the different castles until reaching the final one, where Bowser would find a magical treasure. Again, LOTS of adventure, some spooky medieval stuff, and probably find all the hidden corridors and stuff. Would look good in 4K.
 
People often throw around a concept of Bowser controlling an army aka an RTS or any other strategy games, but the catch is that these games are composed of different useable factions. Mushroom Kingdom is gonna be a faction, and it's going to be the primary one. Such a game is gonna star Mario naturally.