Ditto . I really like ATTWT it feels atmospheric .However there are a few songs that I would be great to hear Steve on . SIAJ and BR are two songs I love but I always imagine how they would sound if extended with a Steve solo swirling around.
It's interesting you say that, the atmospheric end of Ballad of Big is one of my favourite sections in Genesis music, a lot to do with what Tony's picking out on the Yamaha piano. I think Steve could've added some real magic to this sections as well.
As for later stuff, I guess the Tony solo songs I could hear being done earlier with Peter, in the sense of Seven Stones and Time Table. Things like Cul-de-sac perhaps. Probably the hardest thing to reconcile is the amount of polyphonic synths used in the late 70s and early 80s which is really alien to the Peter Gabriel era. If they were done with Hammond, piano and mellotron I'm sure it would be easier to imagine Peter singing them.