I have maintained from the beginning that his final episode is crammed full of strange details that don’t serve any purpose beyond setting up Hemmer’s survival.

They make a point of saying that Aenar have different responses to cold. They make a point of saying the Gorn incubate at different rates in different species, and that they require a certain level of heat. They pointedly don’t show him being killed after he leaves the ship - out into the cold, where the Gorn cannot easily incubate. And those are just the major things. There are a lot of little dialogue bits and production details that on close attention seem deliberately seeded in much the same way things in The Wrath of Khan were set up for a payoff The Search for Spock.

Of course, this could have just been the production leaving the door open to bring back Bruce Horak if they wanted, or to add in a note of hope to soften the blow, but I believe it was a calculated prelude to a planned return.