It's unfortunate the first development team put the new team in this predicament.

Generally speaking, it seems difficult to imagine a realistic scenario in which these classes won't be separated into a healer and the other a nuker. The problem is, neither currently fit those roles completely. This wouldn't be a problem at all to simply design these classes from the ground up. However, Square has to realize that some players leveled CON for nuking and THM for the same reason. So whichever gets cast as the healer, there could be an angry backlash from that group of players who is now stuck with a healer rather than nuker. But I think it's an inevitably and hopefully those players can see that, for the greater good, it was a necessary evil.

Now for some speculation. I suspect CON will be the healer and THM the nuker. The lore that follows CON and the Cure spells seem to fit more closely to a healer/WHM than does THM and Sacrifice. Of course I'm overlooking the nukes and ancient magic available to CON, but for whatever reason I feel the total nuking package is easier to foist on THM and its associated story.