In DFTalk 20 (I think), Toady said that one issue involved in fortress un/retirement is that time jumps to the beginning of the next year without advancing the world or resolving army situations, etc., which leads to weird situations.
As odd as it may be to suggest a change to a release that isn't actually here yet, what if fortresses weren't forced to begin on the first day of spring?
I suppose the real suggestion here is allowing the world to simulate outside of a play mode, from the main menu, which is much more feasible with the upcoming changes to the world making them more active and alive.
Once you generate your world, you could advance time as you please (but only in a forward direction, obviously) and start a fortress/adventure on any day you wish. When you die/retire, you'll simply go back to the main menu and start again from the day you left off.
The negatives here would involve having to wait for tween-game simulation, which could be a significant amount of time (similar in scale to worldgen itself), but the positives would be a much more seamless chronology for the DF world. Plus, challenge gamers could start a fort in winter for the heck of it, if they want.
Perhaps it could do what adventure mode does, and advance time a couple of weeks after you die/retire, to avoid oddness such as reclaiming a fort on the same day you lost it, but just have that timeskip be an actual world simulation period rather than just advancing the date arbitrarily.
Anyway, just a thought. I think it would be a compelling game mechanic, and it would bring a certain amount of finality to your actions, and make the forward march of time much more apparent and 'real' (although I'm sure the upcoming release will help a lot with that.) You wouldn't be losing the ability to start fortresses on Day 1 of a year, you simply have to turn the dial beforehand.