Let me start off by saying that I agree in part with the OP. Having the option to have a guild castle vs a guild airship would be nice. It would add some variety to the setting. And while I like the concept, I don't see this happening. I just do not think SSG has the resources to pull off adding this to the settings. Airships were kind of the "easy way out" when the player base was asking for guild housing. We already had the entry points graphicly in game (the airship point in the market was already there, and I believe was in other parts as well. If its wasn't, its just a simple "cut and paste" graphic in the other houses) so it was a decent and simple access point that could be put in all areas and it would blend in. Just the ships themselves needed to be designed, and they did a great job, IMO.
With the castle idea, where would we put the access points? The simple answer would be having a weyfinder/portal next to the ship access point that would take you to an open area where you can ride up to your castle and enter it (kind of like riding up to the castle in feywild). So as you build bigger castles (like getting bigger airships) you would see the change. Then they would need to design the castles themselves (probably a dozen different variaties, adding walls, motes, ramparts, customizable banners to fly and hang from the walls, etc). This would be a HUGE graphical undertaking and probably be a whole expansion in itself. Would SSG do this instead of new dungeons and gear? Probably not, unfortunately. They have the airships, and I think they would see guild castles as a waste of effective resources.
What I propose is a compromise. If you want castles, what if we take that concept and apply it to something that the player base has been requesting for as nearly as long as guild housing... PLAYER HOUSING!
Players can build their own housing, and the style of the house would depend on which realm the house is acquired in. If you want castles, you build a player house in Eveningstar. Want more steampunk, then its purchased in Eberron. Access points already exist in Erberron in the form of many unused doors in the marketplace. This could be expanded to off shoots like Feywild (put a door in the back of the castle courtyard), Sharn (there is a HUGE area that is built that is not being used) and Ravenloft (sure, if you want to buy one of those houses in town.. kinda depressing, but you do you Mr Edgelord). This is something players have been asking for for years, and would bring in more revenue per player than doing it guild wise as well. Might be the perfect compromise for everyone?