Anyone remember the complete Star Trek DVD box set? It was Star Trek 2-10 standard edition DVDs and the Star Trek I special edition DVD. Then they released the Star Trek II special edition DVD so out came the box set with two special edition movies and eight regular edition movies. Then the Star Trek III special edition DVD gets released and out comes the box set with three special edition movies and seven regular edition movies. This kept going until all ten movies had special editions.
And don't forget the Indiana Jones trilogy with a fourth movie on the way. I can understand why studios release these things, but I don't know why people actually buy them. Who wants an incomplete set? With these Marvel movies though, I suppose you can make the case that The Avengers
and the movies leading up to it constitute a series.