what did the after credits scene of revelation mean?

I wasn't sure myself. The idea that he is alive is possible. One thing I noticed was that Eko's body was never confirmed to be destroyed, only his Yurine. The one thing I was listening for was the footsteps, Eko had a prosthetic leg. But the sound I heard was two steps that sounded similar. Eko, when he walked had a distinct sound, however the film makers may have overlooked that detail... or maybe Eko's body still had both it's legs. What it does is leave it open. If this does well, they don't want to close to door to the gold mine, do they?

Personally, I thought Otoha was the one that showed up in the after-credits scene. (Where someone picks up the boot, right? Just to be on the same page.)

Otoha is the city's current Karas, to replace Eko who had after so many years of protecting it, decided the city wasn't actually worth protecting. I figured this was a hint that the process was cyclic instead of Eko being the *one* Karas who went rogue. Kind of a poke at how bad humanity is, if that makes any sense.

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"Don't take life too seriously; nobody gets out alive."

It's Otoha, if you pay close attention you see the crows feather which symbolizes Otoha, through the entire ending he was sending a feather to all his friends, he was just paying his respects as the new guardian of the city.