First off, welcome to the community, we’re glad to have you here!
In a blog post about the mainboard, Framework has talked about the potential for upgrading to new mainboards
but you should be cautious because there has been no official talk of a 15" or 17" laptop, so it’s all just theory that there would be a potential for upgrading the mainboard into a new chassis.
As for PCIe lanes, it has been said in other threads that the ports are already stretching the 20 lanes on the 11th gen cpus thin. Adding more ports would be a difficult task as they would have to adapt from the 4 ports on the board, and those ports would have to share the 4 PCIe lanes allocated to each of the soldered ports.
All in all, the current mainboard was designed for the laptop that is available now, and while it definitely CAN be put into a new chassis if it was made, you can’t get more functionality out of the silicon that is already there just by making the case bigger. I would say that you should buy the Framework for what it is today, a very capable and repairable 13.5" laptop, and if that isn’t what you need, you should either wait to see if Framework announces a 15" or 17" model, or you should buy a laptop that is better suited to your use-case.