The Summoner is fillimg the Red Mage slot currently but in Heavensward they are suppose to focus on Dreadwrym Trance summoning and harnessing the power of primals. Red Mage is going to be the Melee DoT caster when it gets added and Summoner playstyle will be based around summomning the power of the primals and using their abilities bu absorbing primal aether.