I'd say it's doubtful that any truly original alien could be created for fiction by this point. Minds more scholarly than mine have already proven that it's no longer possible to come up with a completely original plot, as all variations have now been done (the best we can do is put different colors and combinations together). The same can be said for fictional aliens - at least insofar as fictional aliens that can be depicted on screen or on the page in such a way that one can actually tell stories using them. One could come up with original aliens from a scientific perspective - no one had arsenic-based earth microbes on their radar before last week, after all - but while you can make a PBS or Discovery Channel documentary about such things, they don't really make for good entertainment, which is the prime function of fiction.
What's sad is when people go hog-wild about what they think are new aliens (or tech) in a film or TV show, only to find out "oh". The Borg is Exhibit A. When they were introduced in TNG everyone raved about them, and indeed in the late 1980s nothing like them had ever been seen on American TV. British viewers, however, had been watching the Cybermen on Doctor Who for 20 years by that point.
They weren't aliens, per se, but I saw a few comments where people talked about how original James Cameron's avatars were in Avatar, not realizing the basic concept of the avatars (using tech to implant human consciousness into organic constructs to explore inhospitable territory) was written about in 1957 in Poul Anderson's classic story "Call Me Joe", only substitute Jupiter for Pandora. What Cameron did was give everything a different coat of paint, drew in elements from a dozen different films ranging from Dances with Wolves to Aliens, hid the "joins" with 3-D distractions, and ended up with a combination that appealed to the masses. Therefore he gave the illusion of originality.
The closest I've seen in recent years to a fully original alien is Steven Moffat's Weeping Angels in Doctor Who. But even then, I recognized elements in them from other stories and films I've seen in the past.
Alex