Well, to be fair, besides from Stallone and Schwarzenegger a lot of the stars of EX 1 and 2 (Van Damme, Lundgren, Austin, ...) actually work again in DTV productions.
But yes, I also don't think that we will see him in more theatrical movies after his appearance in EX3 (maybe sometimes we will see him in a movie from Robert Rodriguez or Quentin Tarantino again, that would be great). I just would like to see Seagal in this movie, he made a lot of great action movies in the 90s and because of that it would make sense to have him in an EX movie.
I take your point about the stars you mentioned going back to DTV work. However they have all gone back and done something different and arguably better than before, for example Dolph Lundgren and his supporting roles in a wide variety of films. After Seagal made Machete he just came right back to DTV movies with Born to Raise Hell as if nothing had happened and he hadn't just had a film released in theatres. He failed to use that as a way of improving his career and that's what I feel will happen again this time if he does indeed get a role in Expendables 3.
If that role happens it will be because of nostalgia for Seagal in his 1990s heyday. Like you say he and Wesley Snipes are the last two big names who have yet to appear in the franchise.
Seagal will not be called up for Ex3.
He is in hopeless physical shape and being a man of action is now beyond him.
Completely agree with you there he no longer looks like a credible action star like the rest of the cast do.
Even if he can get in some sort of shape I think the best thing would be a cameo appearance similar to that of Chuck Norris in Expendables 2.