As of now, there's a set of randomly generated missions you can go in, smartly called exploration missions. All of them use two default npc races. My guess is that they will just start using player races instead of those two.
Exactly, nothing stops the game developers from randomly picking a race from a database of races, and then plunking that race into a randomly generated mission that any player can encounter. STO must engage in letting players create canon, because if that happens, players gain a sense of ownership in the Star Trek universe, that they themselves created something valuable, and that ownership easily translates into customer loyalty.
"Canon, in terms of a fictional universe, is any material that is considered to be 'genuine', or can be directly referenced as material produced by the original author or creator of a series."
But STO is material
produced licensed by the creator of the Star Trek series, and the STO is in fact 'genuine'. If STO is canon, then anything
inside STO is canon, including your race.
In fact, Star Wars Online is in fact canon, because it is licensed by LucasArts. In fact, SWO documents certain missions that occur during this time (HK-47's rebellion in Mustafa), and these missions are in fact very integral to the canon. What we don't know is if SWO's
Jedi are canon, because SWO has multiple servers, so a Jedi that may exist in one 'server' may not exist on a separate 'sever'.
EDIT: Come to think of it, I want to edit that Wikipedia definition to include the creators' licensing to be canon as well. George Lucas did not write everything in the "Star Wars EU", but he did license it out, and he approved of everything in the Star Wars EU.