Bill Dixon would've been awesome on Ascension though I can see why, in theory, he's not there.
Thornton and Shorter make a lot of sense and certainly could've been on it. Simmons knew Coltrane, as did Lasha, so I suppose it could've worked out if they were in New York at the time of the recording.
The only player I know of who was asked and demurred as far as being on Ascension was Frank Wright, though there might've been others as well.