When it comes to other aliens in the Alien/Predator universe, I tend to go by the same logic that if Aliens and Predators can share a universe because both are from 20th Century Fox movies, then other aliens featured in 20th Century Fox movies have the potential to be in the Alien/Predator universe as well.
Unfortunately, out of the nearly 40 20th Century Fox films featuring aliens, only 2 can be added to the Alien/Predator universe as is:
The Abyss
Solaris
If we can recon the Alien timeline so that Prometheus is set in 2193 and Alien in 2222, I would love to include Avatar in the universe as well:
Since Hollywood is all about remakes, I wouldn't mind 20th Century Fox remaking a couple of their old movies INTO the Alien/Predator universe.
Alien Nation might make a good addition if it's set in the time between Alien 3 and Alien Resurrection and have the Newcomers less humanoid:
And Enemy Mine if only to have the pit monster in the universe:
As for the Arcturians, I feel they should be a race of shapeshifters, mostly because the name drop in Aliens must have been a reference to the 1975 comic The Long Tomorrow written by Dan O'Bannon. It would also help to explain why gender doesn't matter when it's Arcturian. And judging by their appearance in the comic (NSFW -
http://i.imgur.com/fGfbILK.jpg), I like to imagine the Arcturians in the Alien/Predator universe as an intelligent, peaceful, non-virulent, and less graphic expy of the Thing.
Who wouldn't want to get some of this Arcturian poontang?