Jack Kirby originally conceived the idea and intended for the New Gods to be a part of the Marvel Universe. He teased it with Thor #128-129. With Ragnarok being the creation of them, and the Asgardians becoming the Old Gods. These ideas were still part of the Kirby's original New Gods series when he went to DC. IIRC, there was even a panel with one of the New Gods going to what is supposed to be the remains of Asgard (not what it's called in the series) and finds a helmet that looks exactly like Thor's. So in a way, if you want to look at it like this, the original New Godsis kind of like a sequel to Kirby's Thor run.

What my question is, has there been any other scenario like this where a writer intends to have a story, event, or series be part of one company (Marvel/DC), but it eventually comes into fruition when said writer moves to another company (DC/Marvel)?