The Inhumans are confirmed in that role, I believe. Agents has set up and tied together other stuff, but that is clearly a big part of the reason it exists. That said, we are 4 years from that movie, so we will see what goes on. Agents may run is course in another year or two, though I think it will likely continue. S03 week be about the Civil War (with Inhumans instead of Mutants), S04 will tie in Ragnorak, and S06 and S07 take on the Infinity War.