I think Altima will be the third or final boss of the final Eden wing. We'll also learn her real name:
Gaia. The Gaia we've been running with in Eden's Verse is her First-bound fragment. This is the reason Gaia can call upon the "power of darkness", ie: tearing open void gates, using time magic, summoning a cool hammer, etc. Gaia's near-total amnesia, as well as her sudden appearance in the Empty after we reawakened Eden, is directly related to this. And then there's this exchange, post-E7/pre-E8:
Ryne
I'm glad you're back to your usual self, but how are you feeling?
Gaia
Much better now, but something is still bothering me.
The faerie spoke to me in a dream, and a stream of memories rushed through my mind. I don't recall anything like this happening before, and I'm not sure what it all means.
Ryne
What sort of memories?
Gaia
I can't make any sense of them.
They were only brief flashes, seen through someone else's eyes. At least, that's what it felt like.
So, why the third/final boss, instead of specifically one or the other? 'cause in Final Fantasy 8, the ultimate form of the final boss resembles two women fused together by their hips/legs: If Altima is the third boss, then I fully suspect something resembling this form will be the final boss. If Altima is the final boss, then this form will be Savage-exclusive.
As for why Altima specifically, aside from the Virgo speculation? It's actually kind of silly: Altima was a mistranslated name for Ultima back when Final Fantasy Tactics was released. Ultima is a series staple, from the spell to the Ivalice Scion of Light... and the villain of Final Fantasy 8, Ultim(eci)a.
As for the other two bosses, we'll be seeing Erebos, the mentioned but largely unseen voidsent from E2, and Griever. I don't believe Griever will be a Voidsent, however. Rather, I think he'll be an Astral/Dark primal, possibly summoned with Gaia herself serving as its vessel. Also, it will be a primal of Ryne, Gaia, and the WoL's own design. The Lion motif will be Ryne's doing, based on Thancred and his Gunblade, the Lionheart.