COMIC LEGEND: Kurt Busiek planned an Avengers crossover but it couldn't come out because it was approved too late to be topical.
In 2001-2002, Kurt Busiek wrote an epic Avengers storyline involving Kang. The story is typically referred to as "The Kang Dynasty," but I don't know if that was ever the official title.
It was voted into the top ten Greatest Avengers Stories ever told when we did a poll back in 2013. It really is a cool story (any story that involves a giant Captain America fighting a giant Kang has got to be awesome, I say).
However, interestingly enough, this was NOT the first idea Busiek had for a major Kang story (EDITED TO ADD: And, as Kurt mentions in the comments, it was not even his first major Kang story in the Avengers, as Kang was a big part of the 1998-99 maxi-series Avengers Forever by Busiek, Roger Stern, Carlos Pacheco and Jesus Merino, which finished #5 on the Greatest Avengers Stories list, so obviously I should have recalled it). No, following up Busiek's epic Ultron-centered story, "Ultron Unlimited" in 1999 (#4 on that aforementioned list of Greatest Avengers Stories), Busiek had a plan for a major Kang story that would become a companywide crossover. The only problem was one that is quite familiar to Kang...time itself.
At New York Comic Con this past year, Kurt Busiek revealed at a Marvel panel that he had a companywide crossover planned called Y2Kang, where Kang would revert the entire Marvel Universe to the year 1900 at New Year's Day, 2000.
The problem was that Busiek did not receive APPROVAL on the crossover until it was too late to actually get the book out by the books cover-dated January 2000.
So instead, Busiek ended up doing the Maximum Security crossover later in 2000...
I liked Kang Dynasty a lot, so I can't bemoan this TOO much, but it definitely sounds like it would have been a cool idea!
Thanks to Bleeding Cool and Newsarama for their coverage of the NYCC panel where Busiek discussed his plans! And thanks, of course, to Kurt Busiek for revealing this neat info! And double thanks for pointing out the Avengers Forever aspect of the story!