***** Possible Spoilers / Speculation *****
Couple possibilities, this is the one that I like the best:
1. Clairvoyant is a high ranking SHIELD official.
- This would explain how he/she knows almost everything that has happened except Coulson's reincarnation (since the most recent episode revealed that only Fury seems to know about that)
- It would set up a Fury vs SHIELD storyline, which parallels well with the tension seen at the end of the first Avengers movie
2. The blue thing in the jar is
Mahr Vehl - He is Kree, which would probably tie in well with the new GotG and Avengers 2 stuff coming up
- His name is Geheneris HalaSon Mahr Vehl (GH, like in the tv show)
- Skye's body seemed to accept the serum she was given, which means she might be Kree? I don't know enough about the comics to make sense of this one
Other possibilities include the blue thing being a Frost Giant (ties in with the Thor world which has been quite prevalent). Or it might not even be blue and that's just the color of the liquid in the tube.
***** End spoilers / speculation *****