I think it will be a hybrid between Stadium and Showdown. You can build pokemon completely from scratch, or import your own pokemon through Home which you can then easily edit the moves, EVs, and IVs. There would also be pre-built rental builds to start you out with for beginners.
There would be a distinction between these "from scratch" versus owned pokemon. I think the easiest solution would be to give the rentals a "Holo-Ball" of sorts, so the pokeball will give it away whether they were built internally or externally. I was originally thinking the designation could be the Marks/Titles, like it could say a title when either a rental or owned pokemon is sent out. However, I think that would look too clunky in VGC matches. However, players with owned pokemon can still customize their "real" pokemon with marks/titles, rare balls, whether they're shiny, and possibly nicknames. These cosmetic differences would be the game's way of encouraging you to import your owned pokemon through Home. Pokemon created from scratch could not be allowed to be imported into Home or other games.
As for monetization? They could do something like limiting you to a box of 30 "rental" (built from scratch) pokemon, with mtx to buy more space. You could delete pokemon and rebuild them ad infinitum, so you wouldn't actually be limited to 30 pokemon, but rather limited to 30
at a time. Whether this space would also apply to "owned" pokemon, I don't know. Personally I'd find this acceptable, but would much, much rather a flat $10 flat fee or, even better, limit mtx to trainer customization. It goes without saying that they'll definitely monetize trainer customization. That's an easy one. It's just whether there'll be further mtx that I'm brainstorming about.
I think Champions will be the new home for VGC. Getting people to play it casually would gateway and encourage people to get into VGC. That's probably the biggest reason against the idea of a 30 box limited space thing, because then people wouldn't play it casually over Showdown. I doubt TPC would shutdown Showdown... would they? I know Konami had DuelingNetwork shut down when they released Duel Links. It's a strange one, because Nintendo and their affiliates aren't above doing such a thing. But it would just really tarnish and put a bad taste in everyone's mouth if they did that to force people to get into Champions. I want to see Champions succeed on its own two legs. I think everyone does, including The Pokemon Company. So here's hopes they do it right.
Also... how cool would it be if
this guy was the host of Pokemon Champions? Apparently Thorton is really popular in Japan. For those who don't know, Thorton is the battle frontier brain behind the Battle Factory in Platinum/HGSS, a mode that lets you use pre-built rental pokemon. Ofc there's also the OG Noland, but yeah, Thorton has popularity on his side. He would be a good mascot for a mode that pits you against trainers where you each use random pokemon! He'd also be a good mascot for all the statistics that could potentially be available in the game.
This is a long-shot but I'm hoping 6v6 battles are supported, and even better, balance is put into all the problems that have plagued 6v6 singles for a while. All those different clauses Smogon bring into the game, like Sleep Clause and Evasion clause? Actually change the mechanics for how things work so that clauses aren't necessary. Because this game is totally insular from other games, it could have mechanics different than the story mode games. And bring balance to the extremes between hyper offense and stall. And yes, you heard me right, I'm saying it's a longshot if 6v6 battles make it into the game. Correct me if I'm wrong, but I think the mainline Battle Spot challenges have only supported 3v3 singles for a while now.