It's so big in fact Sakurai feels the need to introduce a new Pokemon to Smash within every single Smash game with Pika/Puff in 64, Mewtwo/Pichu in Melee, Pkmn Trainer/Lucario in Brawl, Greninja in Smash 4/Return of Mewtwo in Smash 4 DLC and of course Incineroar in base-game.
Why did I single out Mewtwo from just Greninja in Smash 4? You might be questioning, "He's a Veteran and there's no Veterans in Ultimate DLC so this doesn't mean anything for DLC now or in the future!" Well are you absolutely positive about that? Cause I could argue quite the opposite.
-Sakurai (Talking about Smash 4 and DLC)
Since as shown up there Sakurai planned for Mewtwo to be the only DLC character I think it's very indicative of how he views the franchise. Sakurai could've chosen any series, Any character but he chose Mewtwo? Why? Well Popularity could be one thing but a lot of characters were/are popular he could've chosen K. Rool, Geno or numerous other characters so it could also be that he highly respects the Pokemon franchise to the point of feeling the need to include yet another Pokemon. He's always put a "New" Pokemon in every Smash game so him respecting the franchise is quite obvious.
Also there's that Sakurai not wanting to use any additional costs for Post-Game content which is exactly what post-pass DLC is it's post-post game and what do we know it?
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Sakurai already stated he went over-budget on the fighters pass which is quite curious when you add these two things together. Sakurai has massive respect for the Pokemon franchise and also had Mewtwo be his only post-content no costs fighter I think that could indicate a win moment for Baboom.
So why Baboom? Why him specifically? Well Sakurai always looks at a character for what they can bring to the table and what they could have to make them standout just look at what he said for Incineroar even:
-Sakurai (on Incineroar)
So this goes to show he always looks for what a character could bring and what could make them standout compared to the rest of the cast while keeping them fun and engaging to play as. Sakurai specifically stated he always wanted to include a Pro-Wrestler type character and what else kind of character has he tried to make in the past? That's right a Rhythm-based character (as shown
here) Sakurai tried to include a Rhythm Heaven character in Smash which of course likely would've had a Rhythm based moveset yet it could be transferred to Baboom due to his connection to drum-banging and rhythm.
Baboom would be fun and interesting to the point of very much standing on his own compared to the rest of the Smash cast much like Incineroar.