I think that if we are to do another fort, it should be a part of the current story because Demongate failed to be what it was meant to be. We started this game with the intention of fighting off bloodkin hordes, and not a single one came.
In that case, I might suggest holding off on the planned plot conclusion, which appears to be 'kill the crap out of everything'. An interquel mightn't work that well either, since we know what two of the three major bloodkin players were doing for a lot of their time since Steelhold, though if it's before going to the new continent I guess that frees them up somewhat. If we could get the worldgen seed for the Steelhold saves, would that work for creating a world in .40?
Regarding the shipyards suggestion, I can see at least three factions popping up there; one who support the ship building, another who consider it unholy for any number of reasons, and a third who don't care either way but see a lot of potential for misappropriation of funds. Migrants would initially be engineers sent to work on the Ark, and later refugees when the Bloodkin start taking over everything. Building a ship would be a good megaproject to work on as well and keep people occupied. If we do go with it though, I'd say set it after Demongate on the current continent, since one of the problems we've had is a bit of inaccessibility to newer players, and resetting the canon to before Demongate would leave us with this problem again.