Even then, Toad isn't really anything special.
Sure, he's a tiny mushroom kid, but for the most part when a Toad is playable in a platformer, they are typically no different from Mario or Luigi in what they can do, making them not as interesting. It almost feels like Toad would only be worth digging through Mario's/Luigi's leftovers that they didn't use in Smash (And Peach when you factor SMB2).
Then we turn to Wario's Woods, the "go to" game for Toad because he was the protagonist. Not much interesting things there either. So he can pick up people and carry them around. That's been done.
![Donkey Kong :dk2: :dk2:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
Bombs? Don't we have enough of those already? And walking up walls. Like
that will be used without being horribly broken...
Then we get the sports games. ......what really is there? I only know of Sports Mix giving him a giant mushroom attack/protection as a unique technique.
When it comes down to it, Toad is simply
dull.
At this point, despite not being discussed much, King Boo seems like the best choice.
He's not similar to
any of the Mario characters in Smash in design OR ability set (aside from being able to shoot blue fireballs, though those have homing properties unlike Mario/Luigi).
It also helps that he's sort of a major character, being the main antagonist of the Luigi's Mansion titles.