I read Nerevar sailed to Akavir. What happened to him after that? How long after Morrowind did the Oblivion Crisis occur? Could the Nerevar had played a role in helping quell the Crisis over in Akavir, if he was still there? Is Nerevar immortal like the Tribunal? Where is the Tribunal now and what do they do? The CoC, where is he and what happened to him? Is it possible for the CoC, Nerevar, and the last Dragonborn to be together all at once, or is that beyond question? Is Dragonborn immortal? If not, how did Miraak survive for so long? Who out of the 3 is the most powerful you think?

Nerevar is dead, betrayed and murdered on Red Mountain.

The Nerevarine is rumored to be in Akavir, but there's no confirmation of this.

The CoC appears to be Sheogorath, although Skyrim is ambiguous on this point. However, if so, then the LDB and the CoC probably have met, in a disused wing of the Blue Palace.

The normal answer is, whatever you really want to happen to them. The Nerevarine was only rumored to go there by NPC's who otherwise know nothing else. The same goes for the other every other hero in the series.

Not quite. The Tribunal depended on the heart for their powers, while the Nerevarine is now in a sort of stasis, effectively rendering him ageless and immune to disease. He can still meet a violent end however.

For all intents and purposes, they're dead and they're busy being dead. You can make the case that Vivec is still alive, but the fall of Lie Rock means hes effectively not around for the moment.

If you're CoC wasn't the one who mantled Sheogorath and is a Mer, there's no reason why they couldn't get together at some point. Its whatever you really want to happen.

Unlike the CoC, there is no proof or in-game dialogue that truly dictates the Nerevarine's fate.

As for the CoC, dialogue in Skyrim seems to show that the Champion successfully mantled Sheogorath. He/she retains memories from the events of Oblivion.

Would it not be an orgismic feeling if a future ES game could read save files of all of your games and have all of your heroes in one place at one time battling a horde of evil beings?

They aren't necessarily his/hers, due to the fact that time for the Princes is a bit malleable, and may have retroactively witnessed everything any other character may have achieved.