Here's the thing though, Wheatley's arc as a character was linked intrisecally to the Chell-GladOS relationship, and he was a storyline element in it which helped create and extend the character arcs to their full conclusion. His insertion in any future game would be past the role for which he exists and be stale and non-complex.

Personally, I disbelief that we'll see any kind of Portal 3 at least for a very long time. We've already gotten a lot of mechanics and tools inserted into it with Portal 2, and with the release of the map creator I find it very improbable that the employees at Valve at the current time have any interest in it. I believe they might translate elements of it into future games(Not episode 3 personally, as I do believe they understand the drastic difference between the gameplay of the two series), but I'm not seeing any sequel coming in the next years. Even less so a thematic sequel which explroes the same issues and character arcs.

Of course, all of this also comes from my viewpoint of Portal 2 as plainly a feminist mother-daughter story, with the only male presences of Cave Johson, Wheatley and Ratman as secondary characters in that story. So I see no point in exploring it further with those characters which were very secondary to the underlying story dynamic.