I doubt ishgard will function like the current city states do, because if we add a forth grand company into the mix than all the work done on front lines will need to be thrown out the window and redesigned with four competing teams in mind.

During the Heavensward announcement the different ruling houses of Ishgard were emphasized, leading me to think that we may pick one of those houses to ally ourselves with in addition to our grand company and not instead of.

I got off topic a little, but I don't think ishgard will be a city that a character can start in or join like we do the other eorzean cities.