So what we're going to be doing is supporting everything eventually.
Those things which aren't going to get a GMC update, we'll be working on long-term; they won't be ready by game launch. We'll be starting out with that which has already been published, and as new systems come out we'll be integrating them. Anything that's announced for a release we're not playing at game launch. Many of the things that aren't, we won't be playing immediately just because it's going to take a hell of a lot of time to do (and in at least one case, we're waiting on another game to finish their conversion of something because they have graciously offered to share). The plan is to ultimately support everything, hammered and bent as needed to get them to work with the group. Some things will take more hammering than others. It will all be updated to work with GMC before it ever touches the grid.
We will be building all of the splats into the game world and the story, and keeping the metaplot progressing for them alongside the rest of the game's so that it's mostly seamless when we actually allow them into play.