This story really comes from a question from a reader named Cory, who read in the introduction to the collection of Avengers Forever something about a story called Avengers: World in Chains, and basically wanted to know what it was, exactly.
The progression is quite interesting.
Kurt Busiek and Carlos Pacheco wanted to work on a project together. Ultimately, they came up with an idea that became Avengers: World in Chains. It was announced in the various news places as a new 12-issue series the pair were going to do for Marvel, so when a DIFFERENT 12-issue seres by the duo came out titled Avengers Forever, most folks just presumed it was the original story with a different title.
That was not the case. Avengers: World in Chains is, in fact, a still untold story.
It started off with a simple idea - what if Captain America had never been unfrozen? Using that as the basic starting point, the story would involve this alternate reality, and I don't know what else beyond that (and I doubt either Busiek or Pacheco would like to explain further, because, well, why talk about it if they could possibly do the story themselves in the future?).
So the story was planned for 1999, however, there was a snag in their plans. Another Marvel series started around that time that used a fairly similar premise. I do not know if they ever specified WHAT series, but the best guess (and I believe Busiek confirmed this somewhere - I just can't find the citation) is that the series in question was Mutant X, a 1998 series which told the story of Alex Summers (aka Havok) being stuck in an alternate timeline, with various alternate X-Men (like a vampire Storm, etc.).
World in Chains was considered to be a bit too similar to Mutant X (or whatever other series, if it was not, in fact, Mutant X) to be released at the same time. So with their original storyline off-limits, Busiek and Pacheco came up with a similar, but different, project called Avengers Forever, which told the story of a group of time-displaced Avengers (picked from various points in Avengers history) forced to get involved in a war between Kang and Immortus.
In the final issue, which featured practically every Avenger there was, including all sorts of alternate Avengers, Busiek and Pacheco had some fun.
In the issue, they snuck in some characters they had designed for use in Avengers: World in Chains. I thought that was a cute idea.
After the series was done, presumably the pair considered doing World In Chains as a possible follow-up to Avengers Forever, but you know what happens to the best laid plans of mice and men...
So now, almost a decade later, Busiek and Pacheco have just recently finished a run together on Superman. Busiek is going to be extremely busy the next year on his weekly series for DC, so it is doubtful we will see World in Chains in the foreseeable future, but who knows?