In 1980, as reported by Comics Feature #4, there was a X-Men and Legion of Superheroes one-shot in plan by Chris Claremont and Dave Cockrum.
However, Marvel Editor-in-Chief Jim Shooter was not pleased that DC wanted the fourth company crossover to include the New Teen Titans, DC's best-selling title at the time, as he wanted the crossover to be the X-Men and the Legion of Super-Heroes. This friction led to Shooter's decision to stall and cancel the JLA/Avengers project.
With the Teen Titans being a huge hit, it was decided to turn it into a Titans/X-Men crossover instead. Dave Cockrum decided to drop out of the project.
From Dave Cockrum's interview in The Legion Companion book:
"But there also originally was going to be a Legion/X-Men crossover, and I was to draw it. Then they decided ‘No, we'll make it a Teen Titans/X-Men crossover,’ and that's the one that Walt Simonson drew. They figured that the Teen Titans were more commercial."