I dread to go digging for obscure interviews Yoshida has made, but I do recall something along the lines of "We're working on it, but it has to make sense with the lore first" some time around 2.5 and the lead up to Heavensward, think with Heavensward he specifically mentioned it needing content as well, since even I'll admit it would be a wasted concept in this game... Slap them in for Besieged/Campaign in XI and you've got something bloody amazing though.
After Heavensward though, well one of my old predictions for it may come true;
Ysayle summoned Shiva. In front of the Gration. Displaying for all of Garlemald that Echo users can summon Primals, though perhaps they still need to connect the dots, though it can't be that hard, we had a human turn into a Primal and defend us, I know they lost Cid (and Cid-Lite, unless he went back, still annoyed Nero hasn't been around at all...) but they can't be that stupid without him... Would be a good chance for them to pick up the (abandoned?) plot point from 1.0, where Garlemald lumped Echo users with Beast Tribes for seemingly no reason... Given Ysayle went from "She summoned a Primal and must be stopped!", to basically being our ally despite summoning a Primal... I'm hopeful the Scions might eventually look into that, since it's clearly very useful. I can't believe they're still so dense that they can watch Leviathan nom an "immortal" and not figure "Primals can kick the Ascians butts!", Leviathan was a proof of concept that Thordan went onto prove, come on! The only problem with Ysayle and Thordan VII summoning would be their ambition (mass bloodshed and a dictatorship respectively), if we ignore the whole aether parasite thing.
After Alexander I actually am liable to ignore that, come to think of it... You can't slap a Primal there, tell me he'll completely drain the land of aether if left unchecked, then leave him there for months. It's a point where gameplay and story kind of split. As far as lore goes, perhaps we don't summon Primals all that often, perhaps we summon Phoenix to kill of Elidibus or something, then maybe a few other times when Garlemald makes a target of us for it. Functionally that would be no different than what Lou (granted he was going to interrupt the summon, but then he didn't have the Echo) and Ysayle have done up to this point.
It would be similar to Ifrits Trial. As far as lore/story goes, we fight him three times, but I've repeated those trials ad nauseum at this point, because that's a gameplay thing. FC Primal summoning, I imagine, would be much the same as that, or FATEs and Hunts. As far as story goes, those things should logically only happen once, but this is a game and the lore/story is "broken" for the sake of gameplay. Even ignoring that, they're going to have to do something about the whole aether subplot sooner or later, we can't keep having Primals show up, with absolutely no impact on the world, and expect that point to still mean anything at all... They could easily dust it under the rug by explicitly stating the aether is returned when we defeat them, in which case, what would be the problem with summoning Ifrit and dismissing him after?
I think I'll stick to the idea that "Her children" refers to the Primals, too... Something Lahabrea said in Praetorium has always been odd to me. Perhaps it was just that they're fake gods compared to his "true" god, but I got the sense that they didn't want the Primals... Then you've got the reversal of their approach when it comes to Primals... They gave Allag the means to trap Bahamut (and presumably the other Primals). Why? What was different then? They went from presumably building up an empire from the shadows, and having them effectively solve the Primal "problem", to now throwing Primals around at every possible chance... Add to that Bismarck/Ravana (Extreme) "not being the work of the Ascians" (if we're to believe that brat) and something doesn't sit right with me... Elidibus is putting Primals down now too, through the Warriors of Darkness. Could just be Ascians doing their own thing again, seeing as they have seemingly no organizational power, but I'd hope they noted the fact that Lahabrea fell to a Primal and shelving whatever plan involved the Primals... I mean in retrospect it was clearly a terrible plan, it's just powered us up and cost them a few good men...