If you don't want spoilers for the original films, why not set them in a completely different era? Maybe a hundred years or so in the past... A thousand even!!! The characters don't have to be at all related to those in the empire era and thus a whole new story can be told...
Then you wouldn't be writing prequels to the OT, you'd be writing Tales of the Jedi or Knights of the Old Republic-type storylines.
See above post... :)