Are there any plans for non-dwarven (or fort race) migrants that exist in your civ and mixed civs?
I've been wanting to play a fortress thats kinda like Dorwinion and has a dichotomy between two different species of sentient people working in harmony, but I'm wondering if mechanically that's something that's going to be in the works.
As mentioned by others, you can bring in other races by accepting petitions from visiting scholars and performers, and with luck you may be able to get enough breeding pairs to get a generic base for one or more other races. The luck factor is important, though, because it seems the set of visitors available to your fortress gets determined at the embark, and they're not replaced when they die of old age and foolishly trying to enter the fortress when there's a full necro siege ongoing.
Successful breeding of other races more or less requires that you check the visitors (using Legends Mode info) for spouses that might migrate to the fortress as well, or, for the bachelors, that they're actually willing to commit to hetero marriages (using DFHack, as most won't). Avoid mercs and monster slayers, as they'll never become citizens, and definitely stay away from the bugged performance troupes (which is where most/all elves and goblins are found).