Naturally, there's already a lot of speculation about what that spinoff show will feature, but the EW article has this to say:
We don’t know is which characters from the current series will be moving over to the proposed spinoff – so let the speculation being on that. There won’t be an implanted pilot episode this season directly setting up the spinoff, either (like how CW launched The Flash out of Arrow). However, story elements that are still to come on SHIELD this year will be used to lay the groundwork for the potential new series. So by the end of the season – assuming the details for this project haven’t already leaked, which is rather unlikely – the spinoff concept should be clear.And it's been confirmed that there will be some direct tie-ins between the television show and the movie, not as drastic as the changes that Captain America: The Winter Soldier wrought, but they will be there.
So given those two elements, I think it's safe to assume that the spinoff series is going to have something to do with the impact of Avengers 2 on the Marvel Cinematic Universe. The question is, what could that be?
We've seen all sorts of rumors about some major (female) character who will be introduced. And there were photos of Tony Stark, Captain America, and Thor conversing while a bunch of SHIELD recruits were training in the background (running on a track). So my theory, and it's just a theory and based on a whoooole lotta guesswork, is that by the end of the movie we're either going to see SHIELD reconstituted as a single entity, no longer on the run from the authorities, but no longer the omniscient and omnipotent organization it was under Nick Fury (there is precedent; Tony Stark was the director of SHIELD for a while in the comics, and doing that would nicely set up next year's Captain America 3: Civil War), or some sort of successor organization would be set up along similar lines (SWORD, for example), or both.
Now, in the comic books, SWORD is sort of like SHIELD, but deals more specifically with alien incursions and the like. That would fit in well with the idea that there is now a recognized "alien problem" (first the Chitauri, then the Kree-DNA-enhanced Inhumans, and then the Chitauri-technology-enhanced Ultron*), and would provide a nice symmetry with the parent show in terms of names.
UPDATE: Nope. Looks like the spinoff is going to be about Mockingbird and her ex-husband. Although there's some mysterious third Marvel TV show in the works as well, apparently...
* Not a spoiler, just a wild guess based on what I've seen in the trailers.