The first thing i aim to do is give this link to; cheatsheet/peoples time wasted in mass quantities/poor mans imagination indexed:
https://tvtropes.org/pmwiki/pmwiki.php/Main/MacGuffinNow in regards to your inquiry, the heart of the titan Nyx, left behind after her defeat in the God/Titan war and banishment to Tartarus could absorb all the avatars power and make the sealed titans harmless. The problem is, its beeing protected by a society of old heroes. You see, the heart is an immortal innocent child and the heroes dont want it used for anybodys agendas.
Another option is a shroud once held by the last wife of Hades(no, not the current one). It is said she was created out of his tears shed in remembrance of all the dead. She died of old age, after death her essence fades away, leaving only memories and the shroud. It is said it can hold all the memories of the wearer when a word is uttered in its presence and at one point, all those who entered Tartaros have uttered a sound. The shroud is currently on the back of Charon, getting it a feat indeed.
Zeus never explained how Athena just jusmped out of his head one day. Was it the only time such a thing as happened? What was the catalyst? Or are there more children of the Mind running out there? Could the reason for this miracle actually be repeatable by the titans and if so, could it weaken them to the point their children can be used as leverage against them?
Worst comes to worst, you can always ask the Oracle. The dangerous one, burried under a mountain, her eyes ripped out and held by her captors. The eyes speak you should know, they speak of things better left unsaid. And if they are somehow reconnected with the Oracle, it could spell doom for all. Even the Titans are afraid of Fate you know, even locked away they lack bravery to oppose it completly.