A nice outdoor gas/wood fireplace could be installed and would provide heat and ambiance.
I put heat in a outdoor slab, 10' x 40' for a kennel.
I used a hot water heater ( I think it was a 50gal, 63,000btu), 007 pump, and XL-30 expansion tank in a closed loop system.
BUT!!! you have to start heating the pad early in the season and you have to keep it running, you can't be turning it on and off.
This can work, but you better be rich
Because physics don't matter