The upcoming version of Shane could be from a memory long before the zombie apocalypse.
The Walking Dead has used flashbacks in the past, and the Season 8 finale actually featured Carl — albeit not played by Chandler Riggs — via flashback, so Shane could definitely pop up during a blast from the past.
That said, we shouldn’t rule out Shane returning via flashback rather than hallucination.
We might have to suspend out disbelief that Rick and Shane somehow look younger than they did in Season 1 in these flashbacks, but we’ve been suspending our disbelief to enjoy the zombie apocalypse ride for the better part of a decade, right?