If the floor slab is insulated on the bottom (so you're decoupling it from the earth), and it's around 100 - 200 mm thick, I think it could do some nice passive

If the floor slab is insulated on the bottom (so you're decoupling it from the earth), and it's around 100 - 200 mm thick, I think it could do some nice passive heating/cooling - cooling the space in the summer and warming the space in the evenings. But, it's true any active heating would also be buffered by any thermal mass (especially radiative heat). But you can just cover up the slab with carpet to limit the buffering.