I don't agree with the sentiment we have to get stuck with a Gen 8 Rep just cause. We didn't get a Pokemon based on shilling out one in Smash 4's DLC, we got the most requested veteran (not counting Snake for obvious reasons) of all time specifically for the Smash context as an experiment with DLC early post Smash 4's release. I'm sure being a Pokemon helped him, but he was chosen for his Smash context first and foremost. Sure, we got Corrin after getting Robin in base game Smash 4 as an example, but they wanted a character from a relevant title and that late 2015-2016 period was really rough from a choices perspective. Inkling's complexity probably was a bit too much for a late DLC pick when they were already looking to transition to Ultimate anyway, so they were out. Xenoblade had just barely gotten a character in base game and Xenoblade was in a comparatively tough position as a franchise compared to how much it would soar just two years later. Fire Emblem was officially an up and coming brand with one of Nintendo's biggest releases of the time (attempting a Pokemon-esque two versions was ambitious if ultimately unnecessary), and they just barely got in on the back of Sakurai being convinced by his team on a unique move set. It was a fairly unique set of circumstances that led to Smash 4 adding 4 FE characters at the end of the day.
I think that necessitates a bit more of how we frame this specific situation. They don't necessarily care that franchises have gotten fighters already in the game, but that's not really what I think makes a Pokemon more or less likely. The thing is, Smash is an advertising avenue for Nintendo's brands and their systems that also revels in being this huge crossover. There's a lot to be gained by putting in characters into it, but arguably less so with Pokemon in particular given the brand's huge dominance already. Remember, base game Ultimate added 4 Pokemon to Smash 4's existing line-up. It brought back 3 veterans in Pichu, Squirtle, and Ivysaur, and also added the completely unique Incineroar in a game that only 5 completely unique fighters. That doesn't mean they can't add more mind you, but what it does mean is that Nintendo has already invested heavily in Pokemon for Ultimate. It may be the biggest franchise in video games, but it's worth examining that fact as something that may actually be more likely to result in no additional Pokemon newcomer rather than a Gen 8 rep. Smash continues to give them an opportunity to build their first party brands and embrace crossovers and partnerships with third parties in a major way in addition to occasionally including fan favorites here and there, so maybe that's why they will choose other content for Volume 2 DLC wise. They skipped it for Volume 1 mind you, so there's a chance they're more than willing to skip it again.
I'll never say it's impossible for Pokemon to get another rep because it of course isn't, but I also don't agree with how likely some people make it out to be and specifically being Pokemon doesn't absolutely convince me that its more likely in this instance. The Spirits are of course a massive strike against base game Sword/Shield since they came well after they were already planning additional fighters, and the DLC hasn't actually opened up options all that much since there's a limited number of new Pokemon in them. You can argue the semantics of which Pokemon were included in that Spirit event and you can argue that there is technically nothing stopping them from adding those Spirits to a Pokemon Spirit Board... but that's also completely out of step with how Spirits have been handled across the board. Spirits developed at the same time major content was developed have always been the practical disconfirmation that I've spoken to earlier, and the one time we got a more specific situation was when Three Houses NEVER got Spirits only for us to end up with Byleth and a Three Houses Spirit board.
When I consider the Spirit context along with the base game context and investment Nintendo has already done in Pokemon specifically, I'm significantly more skeptical of Pokemon in Volume 2 than I know many people are.