I know that nothing has been certain with the Sentry since his very first appearance and it is highly possible that he won't come back, ever.

I think that it would not be hard to conclude that he is some kind of avatar of Apollyon/Abaddon, the destroying angel of the Apocalypse.

http://en.wikipedia.org/wiki/Abaddon

Apollyon is a name close to Apollo, the sun god, even if they are different characters.

Abaddon is also the name of the bottomless pit of the Apocalypse ( a void).

This spiritual being has been identified as the angel of death and destruction.

Some theologians believe that he is an angel, others that he is a demon.