I'm pretty certain we will be able to switch allegiances and join new factions soon enough. If you notice, SE continuously mentions the ramifications of changing in allegiances in regards to the new content. Yet there is no way to switch yet.

I'm expecting the next faction to be Garlemald with their starting point being Ala Mhigo. Honestly, none of the GC are much better than the Garleans. I would much rather have magitek. Lol

Ishgard is a good possibility too, they may be willing to open their doors a bit to adventurers after the Stone Vigil campaign. Maybe that's where those big gates will lead to.