Agents of S.H.I.E.L.D. was later, darker, and better in Season 4, with the show using the likes of Ghost Rider and LMDs to explore new storytelling opportunities with strong results. And while ABC may not have wanted a fifth season, there is a lot to be resolved when the show eventually returns.
While there's also the nature of the pact between Agent Coulson and Ghost Rider, the bigger question is just who kidnapped the team. Rather than giving them a happy ending at the end of Season 4, the team is instead kidnapped and taken off-planet, as we see a shot of Coulson on a spaceship.
Space exploration is a potentially great avenue for the show, especially given how brilliant 4,722 Hours was, but the notion of who took them remains a huge question. The organisation S.W.O.R.D. is the most prevalent theory, but was it them or someone else, and just what do they have planned for the team?