I'm not entirely sold on the idea that what we saw in that chamber was an alien. The show is hinting that Skye may be an extraterrestrial already so having the body fluids of another alien save her may make the narrative a little too alien heavy. Perhaps (if an alien) the house guest is tied to Skye's recently revealed past but that may be too strong a coincidence.
The big assumption floating around is that it's Mar-Vell but I thought we could toss around different and wilder theories.
Here's mine:
Lemurian Like an Atlantean, but different enough to side-step rights issues with the Namor studio. Also if the rights revert to Marvel in the near future, they can always retcon him into a full-fledged Atlantean.
Deviant/Eternal Explains the healing power of their body fluids and they are a pretty undesired property within the Marvel universe for potential studio productions. "G" could stand for Ghaur.
Gamma-Irradiated Explains the "g" in "g.h." and the Hulk was said to have a healing factor during Peter David's run on the title. Other gamma-blasted individuals could have similar abilities, possibly in their blood, though Coulson's and Skye's blood transfusions could lead to a She-Hulk type situation.
If they do go the alien route, the house guest could be one of Yondu's people (Alpha Centauri?) for a possible tie in to the GotG movie next season (fingers crossed). Since Michael Rooker is playing Yondu in the film, a guest role on the show next season wouldn't be too far outside the realm of possibility. Yondu could come to Earth looking for his friend and then seek payback for what happened to him.
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Side note: I think M.O.D.O.K. (or he-who-will-become-MODOK) would be perfect for the Clairvoyant. He would definitely fit in this acronym-filled season.
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When I first saw that the magical elixir was coming from someone/thing, my first thought was that the original Human Torch was going to be in the chamber. Years ago, during John Byrne's Namor series, a character named Spitfire got a blood transfusion from the original Torch, Jim Hammond (apparently the android had blood to donate -- just go with it) and it healed and rejuvenated her. For a brief moment I thought AoS might go there with this insane premise (my mind flashed back to that glorious moment in the Captain America movie when you could see the original Torch on display) but they didn't. A missed opportunity.