It's funny. Now that I have that basic idea of Magical Space, a lot of ideas are just beginning to flow.
One thing I definitely want is elves. There are a couple of reasons for this:
1) I want BTR to be able to handle classic fantasy as easily as the big wacky gonzo stuff, and elves are a pretty good testbed for that.
2) They make a good "starter race" for first contact. Being very human-looking, they will be easier for humans to interact with than some intelligent slime mold. Slime molds can come in a supplement, but for the core product, it's good to keep things recognizable.
3) When I started brainstorming setting ideas initially, one of the images that jumped out at me was an elven druid piloting a tree like a mecha and fighting some technological (presumably human-built) mecha.
Space elves, and elves in general really, are a tricky thing to pull off and make cool.
Things that I want to do with Space Elves
1) Bio-tech. Elves have a long association with the natural world. But I don't think the whole "hippy-dippy elves" thing would stand up too well. Especially when you compare them to a technologically advanced human civilization. So they use magical DNA sculpting spells (shape changing spells, but the results are genetic as well as physical) to induce Lamarckian evolution.
2) Magic. Elves are inherently magical creatures. My thought here is to borrow a page from the Burning Wheel RPG. The elves provided in that system do not cast spells, but instead have a number of magical skills that they use to enhance their mundane skills. So in BTR, I'm thinking of including a number of elf-specific milestones that draw on a characters PPE supply to enhance mundane skills.
Magic items will exist, but they do not conform to economies of scale. Meaning that they cannot be mass produced. It is no more efficient to produce 100 magic swords than it is to make 1. Most magic items that exist in the setting will be custom made or civic-scale, like the magical monorail in Eberron. Although the elves will have enough of a history that magic items might even be common to a degree.
3) Spaceships. While there are many fantastical elements in this outer-space setting, this is not Spelljammer. I want to avoid magical "handwaving" as much as possible. I want elven spaceships to have to solve the same problems that human spaceships do (holding atmospheric pressure, moving in space, recycling air) but in ways that fit what's been established about elves. I want them to have bioships that are derived from plants. Elves are not above manipulating animals, but animal-based bioships seem a little too full of squick and elves are elegant beings above all.