Since that mystery is still as enigmatic as ever, of course Clark Gregg couldn’t say whether he’d return as Coulson for any kind of MCU project not set prior to Coulson’s death, either in The Avengers or following S.H.I.E.L.D. Season 5.
Whoever he’s still playing in the final moments of Season 7, perhaps that character will be able to move on to another project in the MCU.
S.H.I.E.L.D. wouldn’t still be “all-encompassing,” after all!