With Sakurai's rule on only having video game characters appear in Smash, they would have to add him in the games first, which I guess they could do via an update with Goh and Chloe / Koharu as new skins. If that happens and Goh does get all the starters, then perhaps they could throw us a curveball and make a Pokemon Trainer version 2.0 with all 3 fully evolved Galar starters be playable (which could be a call back to how the original Pokémon Trainer was planned to have Charizard, Blastoise, and possibly Venusaur be playable before Sakurai ended up changing his mind).Here's my honest theory, I think Goh will catch Grookey too, unfortunately. That's alright though, given Ash already has all six Pokemon, the rest he can swap out occasionally with older Pokemon.
Besides that, if Goh gets all three starters, why not add Goh to Smash? Unrealistic, definitely. Still, it would make sense and would be cool to see, as Goh has become one of my favorite characters in the anime