Bleeding Cool has a nice retrospective on Marvel's Secret Wars, which had me wondering what exactly was Jim Shooter's inspiration for that story? It's well know among comic historians that the series was written to support a Mattel toyline...
...but I don't ever recall reading where Jim Shooter came up with the idea for the series. Thinking about the series and Shooter's previous work, here are some other places where we see some of the concepts that appeared in Secret Wars.
First, The Legion of Superheroes - there are a number of stories which are similar to Secret Wars, including:
The Super-Stalag of Space
The Menace of Beast Boy
Like Secret Wars, this story has the Legionnaires trapped and forced to fight one another. The interesting thing about this story, is that while in the Marvel Universe, superhero vs superhero battles were common place, they were rarely seen in the DC Universe during the Silver Age. Even during the Bronze Age, DC lagged far behind Marvel in this plot device.
Another interesting idea from Secret Wars was how the story was introduced to readers. What Marvel did was in all of their regular monthly comics, they had a weird cosmic bubble appear in Central Park. Heroes would go into the bubble and then come out altered in some way with a reference to some mysterious adventure. For instance, the Hulk went into the cosmic bubble and then returned with a broken leg.
That sort of glimpse into an untold story was pretty novel at the time and really hooked readers on the series in advance. In many ways, it was similar to the Adult Legionnaires story from Adventure Comics which showed Superman walking through a memorial to fallen heroes.
The last story element that may have come from the LoSH was the Beyonder himself. As he was presented, he was an omnipotent being who dwarfed the powers of the assembled heroes. This could have been an extension of the Legion's most feared foe, Mordru.