Hmmm, anything humanoid... Shape can beget at least some outward familiarity. But it doesn't necessarily need to [i]look[/i] the same. 

But the basics would be to think about connection and story. If you can use these aliens to create a story that the players can understand, and if their basic emotions function in a manner similar to the players' experiences, even if it might be expressed differently, then they have something they can connect with. 

It's probably best to avoid giant tentacle beasts or strange globs of jelly or anything too unfamiliar, but if you look at the more exotic animals on Earth and mix and match or tweak them a little, like, for instance, apes that can talk... Giant crabs... I've read about something like manta rays swimming through the gases of Jupiter...