My requested buff would be rather subtle… an upgrade that grants Sentries +25 energy when they first warp in.
While Force Fields don’t really play a role once massive units are out, what the upgrade does do is allow Guardian Shield to be cast the moment the Sentry has warped in, allowing for a more soft-counter defence against things like doom drops, Mutalisks and Battlecruiser attacks. The upgrade is mostly defensive, but it could be used for some cheeky Hallucination play maybe.
My own suggestion has the upgrade on the Cybernetics Core, costing 150m 150g and taking about 85 seconds to research, and requiring the Robotics Facility as a prerequisite (since the Sentry is robotic, and helps reduce its impact with timing attacks). Nevertheless, given the gas cost and the fact it would compete with Warp Gate and air upgrades, such a prerequisite might be unnecessary.