And if Home Alone 4 was successful? We would've had a TV series about it on ABC Family.
Originally Kevin's parents didn't get back together. The ending was rewritten in case they ended up making the show.
In the commentary for the first film, Chris Columbus said that he had planned for Home Alone 3 to be filmed right after Home Alone 2, which would've starred Macaulay Culkin again (Though it is unknown if there would be any difference between the original idea of Home Alone 3 and the one starring Alex D. Linz). He had also pitched an idea of another Home Alone in which Kevin McCallister, now older, escapes from jail and Harry and Marv have since then gone through a Heel Face Turn.
Daniel Stern refused to reprise his role as Marv in the fourth film, saying it was "An insult, total garbage." Day-um.