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...
You could even make them stand alone films rather than a trilogy too... Each telling a different part of galactic history. That way the original films would forever retain the title of "THE STAR WARS TRILOGY!!!"
To quote Obi-Wan... "For over a thousand generations the Jedi Knights were the guardians of peace and justice in the old republic..." Not much said, but more is told in those few words than the whole of the prequal trilogy. Just think of the stories that could be told there!!!
I don't mind the prequels... I grew up with them, and if you watch them as three films they do work as a story... Kind of... But if you try to watch all six there are just too many massive plot holes. Like the fulfilment of the prophecy, both Darth Vader AND Owen not recognising C-3PO, Leia remembering her mother even though Padme dies seconds after the twins are born... you get the idea...
I think, if it has to be told, the story of Anakin's fall to the dark side can be told in one really tight, well made movie. Look at The Dark Knight. An excellent story of a good man succumbing to darkness and ultimately paying the price. You could definitely tell Anakin's story in simmilar fashion.
On the other hand, all we needed to know about Anakin was pretty much revealed in the old films. The back story was, simply that... BACK STORY!!!
Star Wars, The Empire Strikes Back and Return Of The Jedi, are a complete and brilliant story. If George Lucas wanted to make more films then he should have set them in a different era, rather than trying to tamper with such a classic story!!! (Deep breaths...)
This is all just my humble opinion of course... And I would actually love to contribute some ideas for a re-telling of Anakin's story... :)