Actually, we already know pretty well what a 70s version of TPM would've been like... because Lucas wrote the script for it in 1974.
The first draft of ANH has a story structure that's basically TPM in embryo--two Jedi Knights have to sneak a princess off her home planet due to an invasion. It's very different in the details--the invading enemy is the Galactic Empire and the Jedi are hunted outlaws--but it would provide the nucleus of TPM when Lucas returned to it decades later.
The second draft of ANH featured a radical reworking of the story (changing from an escape movie to a rescue movie, as Lucas himself noted) and it was the second draft's plot structure (if not the character relationships) that survived into the final film.
Of course, this first draft has some absolutely terrible dialogue and a completely unbelievable love story... suggesting that, if this script had been filmed, it would've been rather like getting the prequels 20 years early. Fortunately for us, the Lucas of 1974 knew that it wasn't ready for prime time.
Incidentally, before he wrote the first draft, Lucas's notes show that he had a somewhat different idea in mind for his Journal of the Whills concept. In this version there were two major galactic powers, the Galactic Empire and the Alliance of Independent Systems, as well as various small non-aligned worlds--it was essentially a Galactic Cold War. The peaceful desert planet of Aquilae (later to become Naboo) was invaded by an evil "Border System," secretly aided by the Empire. Two Jedi, Mace Windy and his apprentice CJ Thorpe, would be dispatched by the Alliance to protect Luke Skywalker, heir to the throne of Aquilae. By the time Lucas actually wrote the first draft, of course, this whole political situation had been rewritten--and, influenced by The Hidden Fortress, Prince Luke became Princess Leia.