I'd go with either the space wildlife and/or random civilized alien, but not too early in the game's development; I'd rather see them (the civilized sort) if I ever decide to leave human space, or at the very least "unlock" them through some specific quest chain that brings us to a long awaited (for some in the SF-verse) first contact.
Space wildlife reminds me of Earth and Beyond, but given the vastness of space, they're bound to be highly localized, or slowly migratory (why would they migrate anyways?). We'd need to write up a BS but fun to hear/write about space-faring creature biology. Do they live off of minerals? Do they filter feed space dust? There are various extremophiles on Earth as it is!
On the side of civilized aliens, it's gonna end up as humans with a different body structure; given the various ships we have already, it'll be hard to create specialized ships without losing balance, as well as unique weapons and equipment that sets them apart from humans without being too "special". Now that I recall reading a sci-fi space opera novel... Alien ships can go three ways: Visually "extreme", design "extreme, and ability "extreme". However if the ships/equipment problem can't be avoided, it'll always boil down to the aliens somehow having a (hopefully not) a monoculture to present to a player i.e. "WE ARE A WARLIKE RACE, ALL OF US WOULD LIKE YOUR HEAD SKEWERED ON OUR TROPHY PIKES".