I think I would prefer just keeping whatever city or stronghold I start out with. As much as a notion of a Guild Capital might appeal to me. I think that there is a lot of room for this to be pulled off in a painful way. I mean would it be a pre-generated NPC city similar in construction to most everyone else? Or will they be unique blips on the map? Will there be one per guild, with a seemingly limitless number of them (in theory of course). Or will they be a commodity that guilds will have to band together to keep and hold? And what will the immediate benefits of owning one be? A hang out spot, as mentioned previously? I believe having a guild chat would do the same thing as far as communication. As for visibly 'seeing' one another in game, in my opinion, it would be cooler to go to your guildies home-turf and 'knock' on their door.
No problem can stand the assault of sustained thinking. - Voltaire