Current buildings are mostly maxed out, but there is a lot of redesign going on that will add more space by simply being more efficient. A lot of the spaces are now hybrid with 2 teams sharing it on a 2/3 basis. That's for the worker bees though, the C suite guys are getting some premium spots in the towers soonish. If the Hess merger ever closes, there is a giant tower next to Discovery Green that they will inherit too and I'm told it has an exceptionally long lease.
Other options for space that make sense would be the old Continental Building, which CVX had leased about 25 floors for years until COVID hit. Now it's mostly empty, and just a garage for CVX and HPD. It's a nice building though, and it was the Omni HQ building in RoboCop 2 which is pretty cool. The old Exxon HQ is 2 blocks away and has been empty for years, which could also work with some significant buildout. There's also the empty green space from the old YMCA that they never developed, but I don't see them spending 100m+ to do that. That's going to be a decision for the next CEO, which will probably happen next year. The Bridgeland building is currently on hold until some other things are settled, it has not broken ground yet.
I think the "move" is a bit overstated by people looking to shit on California though. The San Ramon campus was sold years ago and just exited officially in June. There is a new building with a 5 year lease nearby that will be the offices for those in California for the next several years. No one is being forced to move right now, and no relocation or early retirement packages are being offered to anyone outside the executive support staff. In the past those were the norm to encourage migration, but for now there is no urgency. I do however, know several people on SR that are actively looking for a new job elsewhere, because they don't want to move to Houston and they know it's coming. I can't blame them, but I think it's a bit early to panic. This is going to be more of a slow transition that ends around 2030.