Would be okay to make a page dedicated to Sonic 3 A.I.R. seeing it's almost an entirely new game but one that relies on data from a ROM (e.g. Doom 64 EX) and has a host of new features added on top of the base ROM data?
Topic on Talk:Sonic 3 & Knuckles
While I'm not opposed to a new page, I think Doom 64 EX has its own page only because Doom 64 was not released on PC. Fan engines are usually on the base game's page, like OpenTomb for Tomb Raider and OpenMW for Morrowind.
What Jeff said. It belongs in "Essential improvements". On the other hand, if the Sonic 06 port ever gets finished, that would belong on its own page.