Mass Effect 3's ending has been hotly debated among fans. Many were simply disappointed. Others cling onto the (admittedly compelling) indoctrination theory. It's clear that many ideas were tossed around at BioWare--and even the original game's writer had his own ideas.
Yet another possible end for Shepard has surfaced through Matt Rhodes, concept artist on the series.
One of the "very, very early" ideas had Shepard meeting his demise at the hands of Ashley (or Kaiden)--the new human Specter. "In this image he has been forced to turn to Reaper technology to accomplish his goals," Rhodes explained in his blog, saying the end would echo Saren's eventual fall from grace.
This potential ending certainly gives the indoctrination theory more weight, as that popular idea suggests Shepard was actually being controlled by the Reapers by the end of the series. Having the surviving human squad mate of the original Mass Effect ultimately defeat you does give the series a surprising "cyclical twist," Eurogamer pointed out.
However, this idea was ultimately scrapped, a decision that Rhodes says was "necessary." For more unseen concept art from the game (including Tali's potential alien face), make sure you read Rhode's blog.