To the best effect with Summoning, abilities like Favorable Winds, Blinding Radiance, Erratic Wisp, Blessed Smoke Cloud, Teleportation, Nether Swap, Apportation, and Chain Lightning. This skill line works well for a stealthy supporting role. Aerotheurgy uses the power of air and storms to build up the player. Specializing in Aerotheurgy will give access to Electric Infusion, Cursed Electric Infusion in the Summoning skill line.
Any of these could make a great secondary skill line for the Conjurer. There are several summoning abilities that a character will only have access to if they also have levels in a particular elemental skill line. One of the first things a Conjurer will want to decide on is if they want to specialize in a particular element. The conjurer already has an ability that would do well with several different playstyles but seems to be built to play a more supportive role in combat.