It would make sense for an ASM omnibus release to coincide with the movie's release because that's when there will be the most hype for Spiderman merchandise.

I wonder volume three could go all the way up to issue #122 (Gwen Stacy death). If that was the case, the omnibus would be a 55 issues, but those issues only have like 22 pages each? At 1210 pages, it wouldn't be too unwieldy, especially is they use thin paper.