So, with the amount of Freeforms out there that can pack every heal under the skill tree, and still do a big old truckload of damage, combine that with a competent healer, you almost don't need an actual tank in Fire and Ice.
But isn't that was Fire and Ice was designed for? To promote the Trinity of Tank-Healer-DPS and teamwork at the same time? You know, like an actual Raid, and not just content you can lolfaceroll through.
So, up the difficulty a bit, make it where you need a dedicated Tank on Frosticus and Kenina, and a dedicated healer on each tank, and if DPS pulls aggro, they should either shed it or go squish.