It's hard to say what direction smash will go in next, but I highly doubt the dev team and Sakurai (or whoever else might step in as director for the next iteration) would need to downsize the roster that much. Thirty characters is just 5 more than melee had almost 20 years ago now, and it's likely to be a third of what we'll end up having in Ultimate. I think somewhere between 45-60 characters isn't unreasonable for Smash 6. That way, they can handle balancing needs for certain characters better than they can now with so many in Ultimate, and they can invest a bit more into the single player modes (getting a good adventure mode again would be great).
They'll really just need to market the next game in a much different way to have enough to offer for fans to be appealed by it despite the likely cuts in the roster.
As for your comments on the species, some of their moves look aesthetically similar, but many are pretty different and unique at this point in all honesty. The strengths, weaknesses and playstyles are overall rather distinct from each other. I'm curious what more specifically you'd like to see changed about any of their movesets? I do agree that Krystal would be a great addition though, and after that, Star Fox representation in smash can be considered complete imo.
I am going based off of what Sakurai said--he mentioned how the original plan of Ultimate would be a complete redesign, and would have resulted in 30-ish characters. There are very few ways to follow up Ultimate--if you offer a similar graphical and play experience, but less content, most people will not want to buy it, even if the new characters are amazing (and outside of Sora and Crash, there aren't that many heavy hitters left). The only way I see them following up Ultimate is having a complete graphical overhaul and a new way to play, to bring in those who are not fans of the current style, which would require having a more limited roster.
Of course, Sakurai says things, and then things sometimes change. I personally don't want a reboot, I want Ultimate to be a games as a service where it just continues as long as possible. Sakurai has said Vol 2 is likely the last for Ultimate, but things could change depending on how the sales go. But in both cases, I'd like to be prepared for either possibility and think through the implications. If Ultimate can't continue indefinitely, the next best thing, for me personally, would be to include as many characters as is possible, like you said. But based on what Sakurai has said, and the financial reality of how that would sell, I doubt that would be the direction they go. Ultimate Plus or Ultimate Lite plus an amazing Story Mode would be sufficient to me, personally.
As for the semi/pseudo-clone status of Star Fox characters--I understand there are nuances in how they play, but I'm talking on a purely big picture level, how it looks to someone uninitiated. To me a truly unique character is someone like Bowser and Bowser Jr--there really aren't any similarities I can think of at the top of my head. Likewise, Krystal would primarily use a staff, and have ice powers, something very different from other Star Fox characters. The same is said by those uninitiated with Fire Emblem--they mostly use a sword and have a counter. I'm less enthused about characters that have similar abilities, but with slight differences in jump, speed, etc. I like characters that are overall different. With Star Fox characters, each one has a reflector, a blaster, a side dash attack, and a fire based launch recovery. I'd much prefer they mix it up more than that in a limited roster, even though there are technical differences such as Wolf going at an angle for their side special. The same goes for all of the clones, semiclones, and pseudoclones (based off of Smash wiki: Chrom/Roy/Marth/Lucina, Daisy/Peach, Pit/Dark Pit, Samus/Dark Samus, Simon/Richter, Mario/Dr. Mario, Pikachu/Pichu, Toon Link/Link/Young Link, Ness/Lucas, Falco/Fox/Wolf, Ryu/Ken, Isabelle/Villager, Captain Falcon/Ganondorf). The worst offender is Ganondorf, particularly since they're not even a part of the same series as the character they're based on. The only exception is echo fighters--I'm fine if characters like Peach and Daisy share moves, so long as they're in a specific category.
Of course, the issue is, and why we have so many clones/semi-clones to begin with, is they take less time to make, and so it's a way of adding more characters than there would be otherwise, and semi-clones are more preferred by fans than full clones. Which is why I'm fine with echo fighters--it's just Ultimate has been inconsistent in the definition (some echoes are identical, while others have major differences from the character they're based on). I'd just expand that category to include all clones/semi-clones, and have one per character. It makes the roster aesthetically better to me, and a more limited roster is an opportunity to pick out the most unique characters from each franchise.
I'm not sure how I'd change the Star Fox characters, personally--I'm not as familiar with the series, so I don't know what moves/weapons/etc from the home series could be used for the characters. But Krystal, based on what we see in Ultimate, is far more unique and has a lot more potential to differentiate herself from Fox, than Falco and Wolf have, and in a rebooted Smash, I'd prefer if they do include Falco and/or Wolf, to make them as unique as Krystal would be.
If I had to be honest I don't really have much gas left in the tank or a drive to push and rally for other Rare characters.
It's not that I don't want them in, It's just that King K. Rool and Banjo-Kazooie took a lot of years and a lot out of me and they always gave me that gut feeling that eventually they were going to make it in because of their insanely high demand, push and to a certain extent importance.
I just don't see that with the other characters, Sure I hang out sometimes on the Dixie Kong thread and while I am pulling for her, DK, Diddy and hell even Chunky Kong were always my favorite kongs and I usually just hang there to talk about more about Donkey Kong franchise as a whole rather than Dixie.
I kind of promised I would retire my account after Banjo-Kazooie got in and I almost did, But I decided I'm making one big final push for my "Current" most wanted character and that's Crash Bandicoot as he's really the last big 3rd Party character that I can really get behind that has a legitimate shot at making the roster, Kind of wished that I voted for him too alongside K.Rool and Banjo-Kazooie on the 2015 Smash ballot, But was so focused on the other 2.
If Crash Bandicoot makes it in then, I pretty much got everything I could ever wanted and can pretty much retire from Smash speculation.
Personally and hopefully I'm looking to make it 3 years in a row.
2018: King K.Rool
2019: Banjo-Kazooie
2020: Crash Bandicoot?
I got my dream character, Banjo & Kazooie, so I'm satisified, regardless of how the rest of the roster comes out. Though, my number 1 pick now is Crash. Much like how I wanted Banjo & Kazooie to complete the Nintendo character line-up for me, I'd like Crash as Mario/Sonic/Crash are the iconic trilogy of the 90s, and I'd like to see that complete. Plus he'd be a great character, and iconic enough to warrant their inclusion.
Edit: Edited to change the Pit/Palutena contrast with Bowser/Bowser Jr. Despite similar origins, I can't think of any similarities in their moves.