I read another article whose interpretation is that Bobbi and Hunter might not actually leave Agents of SHIELD to do their own show, that they might be on both shows at the same time. Maybe it's a prequel, or something that would go between seasons of AoS. So it wouldn't be a spin-off because they wouldn't be off AoS. I still think that might be a bit of a narrow way of defining the term, though.
Is it just me, or does this new season seem to be pretty crowded. I hope the focus is still on our core characters, but if there is a Mockingbird show I hope that the characters leave Agents. There can be too much of a good thing.
I think Marvel Studios must be thinking the same thing. And with Agent Carter doing good enough maybe they can provide ten week blocks of shows for ABC rather than the traditional 22 weeks on and 30 off