I understand why aliens were human-ish back when actual people had to wear the costume and attack Sigourney Weaver. I appreciate that. But now that creature effects are usually digital (which makes me a little sad, actually), we are not limited to prosthetics that must fit onto a human actor's body, and the xenomorphs need not so resemble us.
Here are some basic animal body plans that have been rather overlooked as alien schematics:
Radiata: animals with radial symmetry, like jellies
Molluscs are animals with soft bodies and hard shells, such as clams, snails, and octopuses. I think tentacles are pretty standard alien protrusions, so I'll go with a shell-fish-style alien.
And don't feel limited to animals. There are plenty of bizarre living things in other kingdoms. Look to ye plants and ye fungus, perhaps even ye bacteria.
How'd I do?