My ideal scenario would be to have Shin's adaptations cover 1) What's implied in the film (5th form, flight), 2) What's (mostly) seen in concept art/early production material, and 3) take from the other Reiwa-era Godzillas within reason (so say no magical teleporting from Snow or Red Dust unless he encounters the material/particles needed to make that jump). His adaptations should be enabled to be a one-stop-shop that has properties of all things Reiwa Godzilla without removing Shin specifically, because to quote the movie, "Do as you please." And I don't think it's wrong for me to say that this is what people want Shin to do--crazier stuff.
Unfortunately, Greyshot is... As GvR pointed out, ridiculously restrictive about this. As much as I appreciate him being leader, Shin and Greyshot do not mesh together at all. I get a part of that was us waiting out for a sequel, but I think between Shin Ultraman not showing any signs of previous continuity and Shin Kamen Rider being a thing in the near-future, it's a safe assumption that the Shin series will be standalone reinventions of classic material. Therefore, I think Shin needs to be looser to survive; if not, I'm going to have to push for Ultima over Shin. Even if we were restricted with what Ultima can do, Ultima holds enough natural utility that you can do crazy stuff with.
The unfortunate byproduct of having a character that can "do anything" means that the rules of what that means is left to the devices of the imagination. For Anno, I'm sure he wanted people to envision what they see fit. But in the KWC--as it currently stands--that just opens up a whole can of worms of people "being able to make up their own Godzilla" in a sense. At least, that's the vibe I get from it.
Had things been any different, I would've liked to have both Shin and Ultima--but fact of the matter is, both fulfill the same quota of "the evolving Godzilla". You have Shin, who within his own context, was already highly restricted to begin with by Toho (which that mandate bled into Greyshot's mentality, hence why we haven't been using the production material as we have with other monsters); then you have Ultima, who not only was a crack-ton more relaxed in terms of studio interference (practically nonexistent, with only like one or two stipulations), but has been a natural all-encompassing Reiwa Godzilla in of itself. You have Shin's evolutions, Godzilla Earth's terraforming / ring-inspired atomic breath, and weird dimensional particle stuff of Snow Godzilla with the Red Dust.
With how it stands, I think Ultima is the far better choice. He requires less assumptions of how his abilities work, whereas you have to make large leaps in assuming for Shin. While the vagueness for Shin's powers can be more creatively free under any other circumstance, the KWC don't allow that, hence why he has been far more frustrating to deal with in a KWC context. Keep in mind, he only got in by a popularity vote... Had I had the foresight of what was to come, I seriously would've just opted out of Shin and waited for Ultima.
Shin Godzilla, both the creature and the movie, are some of my favorites in the franchise, and I will still love them to the end. But in the KWC's context, he's probably my least favorite to write with because of the uncertainty factor.