Lack of flying enemies and need more variety on them. In F3 was only the bloatfly, and they only float ground level, not fly.
Even just a few locations (or even just one) ~only reachable by Jetpack, would be enough, and justify it.. especially if it has to be repaired to work.
They could even set it up so that it is depleted on the first flight (IN), refueled, and depleted once again on its flight out ~and lacks the power to carry an external tank of fuel out with it.
(Though obviously it would be more fun if we got to keep and maintain it.)