It would be nice if we could add and replenish crews in space - using either shuttle modules or a special crew shuttle.
Replenishing crew casualties on space stations guarding jump points would become MUCH easier, because currently the only way to do it, at least to my knowledge, is to tug them back to the colony. Another possible application would be to transfer crews between ships in fleet, for instance, when one ship is badly damaged and has to be left behind for repairs, while the main fleet could use few extra hands due to casualties but can otherwise press the attack/continue with their mission. Another possibility would be to rotate crews on long duration missions, though there's already a recreational module for space stations for that particular case.
Edit: Another use for crew shuttles could also be to transfer survivors from one ship to another - for instance when the first ship on site picks them up and then later transfers them to the dedicated rescue ship or colony ship when it arrives, in order to avoid overcrowding.