Since it has been indicated moving fortress sections and boats would be based on the same system, larger moving constructions doesn't seem that unlikely. It'll hopefully only be limited by what Toady and Threetoe feels makes sense and whatever any future magic would allow (Threetoe has that one story about a flying castle after all ;P)
It's hard to pick any one planned feature really, there's just sooo many of them to look forward to. The updated mechanical system as well as proper sieges with diggers/siege weapons and better AI would probably have the most direct impacts on gameplay, and I bet it's going to be a lot of fun to play around with.
The biggest one though I'd say will be what's starting to show in the next release, with "activating the world" and all. Having yur game not limited in any way to the confines of your embark and getting visitors to your tavern, sending out you own caravans and sieges, managing your surrounding hill dwarf and deep settlements etc, that's going to be HUGE
Then there's the whole procedural cultures/races/organizations/general content which'll make the game a whole lot more interesting and different between each world.
An actual magic system, sphere lands and different planes of existence (ie afterlives, elemental planes, dreamworlds etc).
Also not to forget everything adventure mode, especially in conjunction with dwarf mode as they get further meshed together. The whole cycle of potentially playing an exploring adventurer, fighting and learning and making a name for yourself, then finding the perfect site out in the wilderness and heading back to manually pick out an embark crew. Then playing the journey there, fighting off beasts, starvation and whatnot, and finally arriving and swapping over to dwarf mode. Then after a few decades picking out one of your dwarf youths to play in adventure mode and completing the cycle ^^
Someone else is bound to soon point out something else major I've forgotten of course