I actually do believe that a third fairy (or other summon) will happen eventually, just because adding more summons to pet-enabled classes seems to be the thing to do in games like this.

Helios and related ideas are also very old speculation. Been beaten to death on forums already.

The main issue with a third fairy is SE actually making it useful without making the other two obsolete. Eos is already borderline irrelevant past low-level, while Selene offers a marginal benefit, but is boring as hell. End-game parties rarely need more heals or more mitigation, and any DPS-boosting fairy more powerful than Selene would place the latter on a shelf. A fairy that DPSes like an Egi would not only infringe on SMN territory but be extremely redundant since SCHs have no issues at all with solo DPS.

They're probably so concerned about unbalancing SCH that, if anything, they'll err on the side of giving us a very useless third summon.