Nah, all those stacks of Eikondrite aren't useful after all.
They have very very very very rarely made an item/currency that was previously obtainable, used for stuff added later.
I'm sure there are more but the only thing I can think of is Segments in Odyssey.
As such, I've always been extremely skeptic about further use of Eikondrite. One further point to add to this thing, is that Eikondrite stop dropping if you no longer have a Stage1 Prime Weapon in some of your repositories.
Then again who knows? We'll find out soon.
I think they're gonna add Eikon-something to basement chests/bosses, and that it will be used for Stage 3.
I no longer think they're gonna release a new event dropping stuff for Prime Weapons stage3 and further, but that they're gonna add new drops/KIs/stuff to previously released content (like Sortie!)
I'm just hoping they're gonna add a trade-back system though.
Like trade 1 Eikondrite for 500 gallimaufry??
Trade 1 sapphire for 2500 Galli?
1 stone for 15k galli?