A gym would be stupid IMHO for several reasons. I am no fan of fitness centers, but when urgent needed, I would do it at home or - for longer trips - at the hotel. Using a gym will in my world need a shower afterwards. That takes time, and you need to plan to bring changing clothes. And then continue with wet and stinking clothes in the carry on….
But they could put up some massage chairs.
I agree with most others: wifi, catering, space, cleanliness…. And lounge at relevant outposts. How difficult can it be.