You are correct in assuming that WotC itself can't do anything with the product that is BG3. The Intellectual Property within it belongs to them in all legal senses, so they are the only ones able to make new content using them in an official capacity, but the game Baldur's Gate 3 and the engine on which it runs are legally owned by Larian Studios, so they are the only ones who can make any dlc or changes to the game unless they willingly bring in another studio to assist them.
At this rate, I'm not even sure that waiting a few years and then opening discussion with Larian is going to accomplish anything. Given that they're a completely independent company, they're fully making all of their decisions on their own without any external company guiding them. The only person making executive decisions for them is Swen, and he's made it pretty clear that his entire philosophy towards game development doesn't gel with the concept of continued support over an extended period of time. He wants him and his teams to be devoted to a project and pump their passion into it as best as they can, and then when it's in the hands of the public, they make sure it's stable and then call it a day and move onto the next project. I'm sure there's things that even Swen himself wishes could have been in the final version of the game, but he seems to cleave to the idea of "we're proud of what's there and that's good enough for me".
I think the best strategy is to simply be as loud as possible in the present moment before it's too late. It's highly unlikely we'll ever see some of the bigger changes that would benefit this game, as those are shreds of ideas left behind in the development process. I'm going to personally be submitting some things directly through the Larian feedback platform regarding smaller changes that could still feasibly be made in upcoming patches before support of the game is concluded. Beyond that, if things remain unaddressed, it will be in the hands of modders to figure out how to substantially alter the game. Larian certainly hasn't made it easy given that the modding tools are more akin to tabletop dnd homebrew than actual modding tools for a computer game, but people are persistent, so I'm sure we'll get something eventually. I wish I knew the first thing about computer programming so that I could make some mods myself, but alas...
