In coming up with aliens we face a fundamental problem: we haven't met any yet. I have no doubt there are beings beyond comprehension out there in the Universe. But for now we have to make stuff up for our role-playing games and they aren't going to be any help. As humans go our minds they can't create something absolutely new. Rather we synthesize something out of all kinds of facts and images we have lying around. Many aliens fall into one of these categories:
1) They look like use more or less. This happens a lot on weekly television shows. Makeup or CGI is hard. It also happens when you're more interested in telling a story than science. Some of our most enduring alien characters fall into this category, such as Mr. Spock, Lt. Worf and that Twilek dancer at Jabba's palace. Perhaps it's because we relate to them so much easier than an oviparous xenomorph that bleeds acid and favors basic black exoskeletons.
2) They are driven by a single over ridding concern that shapes all our interactions. Okay maybe the Predator was on vacation for the movie and had he survived the future governor of California he'd have returned to his planet and his job being an office manager. Basically anything concerning their society beyond the story is ignored. I'm sure there are Klingon accountants and Wookiee gynecologists but we never see them.
3) The alien looks like an animal human hybrid. I have to state my bias for cat girls in spite of never having owned a cat in my life. Some hybrids are just fucking cool. Humans being humans, we will probably decide aliens resemble something if we encounter them whether it be cats, toasters or sentient shades of blue.
Better writers have spent 100+ years trying to make their aliens alien. Here's some trope repellent.
1) Looking like us doesn't mean you act like us. Gross similarities may fool people into thinking an alien is human but the aliens will eventually give them a surprise. Maybe they're even wearing a disguise!
2) We aren't sure what they look like. Arthur C. Clarke spent a good deal of Childhod's End detailing an alien occupation of Earth and we spent decades not knowing what the Aliens looked like (with a great reveal because it's Arthur C. Clarke!) You can make the exact appearance of the aliens a mystery. Let your audience work up their own idea or simply remain mystified.
3) Having an overriding concern is harder to justify than you'd think. An advanced culture will probably have all manner of jobs. I know I'm talking about human cultures here but that's all I have to work with. Humans have a lot of variety as well. We've produced Steven Hawking, Charles Manson, and Buddha. Don't be afraid to show some non-standard aliens even if they're mostly from a planet of hats.
4) Animal hybrids are okay by me. I'm a long standing fan of the Aslan and Vargr but the Aslan and Vergr have already been done. I'm not denying cat girls to anyone who wants one but if you're going to have an animal hybrid type pick a less used animal for heaven's sake. Make your warrior race act like honey badgers or base your mystics on penguins. Change stuff up. There are a lot of animals that most people know little about not to mention extinct species we know next to nothing about. That's lots of room to invent. That's without even touching the cryptids and paranormal creatures reported.
5) Hang a lampshade on how similar your alien seems to an animal and then add some dissimilar traits. Yes, my character Riasi looks like a cat/human hybrid. However, she has a prehensile tail which might surprise some people. Also she swims just fine.
6) Establish a central trait for your alien then break it. Logical Vulcans being driven mad during their mating time. 'Nuff said.
7) Make your own aliens. We may move quite a ways into the galaxy before encountering intelligent life. We may be alone. Uplift some animals and transform some humans. You can give them alien traits but they still will have some traits we can relate to since we are designing them. The old Buck Roger XXVc game ran with this populating most of the Slar System with genetically engineered sentients.
8) Be original. Try for a very alien look or mindest. GDW did jut this with the Centaurs and Hivers. You needn't settle for a human in s rubber suit.