With 2x12's reappearance of Sif, and the appearance of Vin-Tak, and how it dealt with the "Inhumans", his hunt, so on, I couldn't help feeling cheated. So I had an idea for how the latter half of S2 could have been.
While SHIELD are dealing with the aftermath of the Terrigenesis, elsewhere a Kree arrives on earth, and begins his search. quarter way into the episode, we cut to New Mexico, where we find... DARCY LEWIS, in her own van, with equipment and she stops by an armored figure... who she greets with familiarity... (Sif) Some more stuff, blablabla, Skye in Quarantine... then end-of-episode scene (five minutes or so), we follow an unarmored Sif with Darcy as backup, and find she's following the Kree, only to loose the fight, and the scene ends with Sif asking who Darcy is, and who she is, and Darcy in response, well, pulling a phone out, dialing a number, and saying, "Hi, iPod Stealer... it's darcy..."
Sif is in the Medlab at the Playground, with Darcy and Coulson outside, Darcy explaining what she knows (which isn't much), how Sif was following some guy, and how she managed to be around when Sif arrived in NM. Mention of "What is it with Asgard and New Mexico" has Darcy try to explain about how a thousand years can shift a "something something weakpoint something" from wherever in europe (hence the norse) to New Mexico (which has no local norse mythos), hence why generally, the Bifrost opens there, though it can open elsewhere on earth.
Darcy leaves, no answer as to why or what she'll be doing, but this is her temporary exit from the show.
SHIELD and Sif try to find the Kree, and slowly, Sif begins to remember things - though 'childhood teachings' never left her, her life is returning in drips and drabs. They catch up with Vin-Tak, who explains his mission, and it strikes a chord with Sif, but she doesn't know yet. Also, he cannot restore her memory, his hammer was designed to permanently erase specific knowledge/memories, and a side effect is a temporary near-total amnesia... she will remember who she is, along with events of her life until her arrival on earth. She, doesn't take that explanation well... By the end of the episode, Vin-tak has "escaped" to continue his mission, while Sif now knows who she is, though many events of her life is still gone. She does remember Coulson - both from dealing with Lorelei, and the previous meeting, but most of her adventures away from earth are still blocked. (it's a whole,, familairity helping remember thing).
She decides that, for a short time, to leave the chase for Vin-tak in their hands, and supposedly returns to asgard. End-of-episode, we see her away from the group, spying as they go on a mission... and she mumbles about skye (she knows something is off? that skye is one of the 'inhumans'?)... though only Fitz and Skye herself know about Skye's emergent abilities. (those are dealt with next episode)
Sif Returns, to help Team Coulson. her memory is restored, with a small blankspot of how she found Vin-Tak in the first place, along with the fight when she lost her memory. She lets them think that she only just returned, rather than having never left. (recovered her memories on her own). She has returned to give the team assistance with chasing down an Inhuman who is revelling in his destructive abilities. After awhile (halfway into the episode), she reveals to Coulson alone that "Heimdahl" had informed her about Skye AND how being with the team has helped the girl gain control. She will, not interfere, unless necessary, trusting coulson. (again, not heimdahl, but from her own observation over time. But this is not hinted at in the episode). At the end, Sif ends up having to trust Skye directly, when their target in question does X-y-Z, and Skye has to use her abilities to, counter it...
Vin-Tak returns. SHIELD learns of a "handful" of murders that they suspect to be inhumans, given that people around those incidents report memory loss. The most recent one helping them track the Kree.
Vin-Tak ends up being saved by Skye, who tells the knowing kree that it doesn't matter what she can do, only the why. other than that no ideas. Sif finally returns to Asgard...