Actually, there was some content originally planned for D2:ED that was moved to the add-on, but the add-on itself was planned in advance.
Oh, I'd assumed it was just the unfinished-and-unreleased Chapter 4: certainly the ending of the original ED was a bit, "er, what just happened?!" I vaguely recall the stuff about the outskirts of Aleroth being planned and then scrapped (and there's also a screenshot of some countryside featuring a signpost to Verdistis, but I presume that was just promotional material for ED's release) but hadn't realised any of it was for an add-on. Was that another thing mandated by the publisher?
