If you want a little height added, just make a mortar bed like you do with shower pans before they are tiled. A layer of cement board would be ok if you could fasten down but that may not be as easy as a mortar bed.
Put the bed on 6 mil plastic which is a similar concept as the shower pan liner but not as thick which will help with any moisture and make removal of the mortar bed easier someday. The mortar bed is a fairly dry mix that sets up slow so it is easy to screed and trowel out. They also make an electric radiant heating pad that goes under tile for heating purposes. Seen them do it on the brand x show(hometime).
By the way, seen lots of tile put down in new construction right on top of the plywood subfloor. Would I recommend doing that? I don't know cause a layer of cement board couldn't hurt for stiffness.