I'm planning to add some various creatures, subraces, workshops, and adventure mode goodness. The idea is to play regular DF until destroyed by siege (as I plan to add a large number of siegers, including a weak one that can attack at nearly any time, ranging to ones that only go after the wealthiest of forts but feel like a dragon invasion.
Once you finally fall, the goal is to then reach the land as an adventurer and steal a bunch of loot. In Fortress mode, you'll be able to spend resources on things mostly useful to only adventurers- the Rune Workshop, for instance. You're able to take certain gems and make runestones with them- an adventurer can interact with runestones to give them new abilities. Not always good. I'd love a way to randomize what gems give what per run, but that might not be possible.
I'll also expand races, at least somewhat.
Dwarves:
Dwarves don't really change much. I may add a caste system similar to Masterwork, but even if I do it likely won't be as widely ranged.
Humans:
Humans are another race that don't change a lot. They won't even get castes, and will likely serve as the vanilla-est of the vanilla.
Elves:
Various subraces of elves, with varying degrees of tree-loving and hostility to dwarves. A few races don't always seen elf-y, as one in particular doesn't care about trees, is hostile, and are tough. They don't use metal, however.
Goblins:
A few subraces, including Hobgoblins, which don't attack until you reach a certain level of worth but are stronger and more advanced than average goblins.
New metals. Some aren't available to the Dwarves. All metals will need a specific forge, the Dwarves lack one of them. The one forge they lack isn't the one that produces the best metals, by far, and the dwarves have no use for it- it's an Elven Forge. It allows them to use specific metals that aren't anti-magic, however, is only used by the elf races who care less about trees.
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I'm putting this here because I don't know how far I'll get if at all into this.