This theory is 100% wrong, butI was thinking about possibilities of very distant future content, and considered that Aeternam is "sharded across time", the only Time Mage we know (I'm convinced that Aris is one of her shards), and some conspiracy theorist connections that are similar.
The poorly articulated idea is that basically, in 50 years when we prove to Jas that mortal life is necessary, and once the current Divine Conflict ends, given that the Stone is shattered, Jas imbues a bit of herself into those shards, and uses the mass amounts of divine energy within herself to magic up some stuff that lets her live throughout time as Aeternam. Side effects may include forgetting that you were an Elder God.
Aeternam meets us at the end of our life, and we first met her at the "end" of her's (balance elemental), so concept-wise, I think that the "end of our life" (which won't be literal, because then the game is gone forever) is Jas pulling some Guthix-like thing on us "ok, here's some magic powers. you're no longer your old self, here's the new you. Haha. the old you is dead", or some other variant thereof.
There's some other stuff that seems comparable, but I am so tired right now, I don't have the energy to write it out, so just plug stuff in on your own and have fun with it, even if it's totally fake.