I am sure that we would get the best Flash Gordon ever. But it won’t strike the imagination like Star Wars did, the basic overall plot of flash Gordan is just not the same. Plus pretty much everyone that is likely to see it had at least a little passing familiarity with FG and that will taint the film before it ever gets finished.
So it may be successful but it won’t be the monster hit that Star Wars was. And it won’t get a sequel as for the most part studios didn’t do them very often back then.
Add to this that FG being a “known” product will most likely result in a more traditional studio deal and less toys and you end up with Lucus being in a position that is not as strong so he can not control his destiny as well. So while we may still see other movies he was involved with such as Raiders it will not be the exact same movie as the studios will have more say in them.
So all in all I think you will see a lot of changes and most of them are not exactly good.
Star Wars is not realy the product you want to mess with if you want a “better” result, For that I would suggest that you mess around with Star Trek the motion picture. (Which will be a TV reboot if Star Wars is not made). As that movie had the potential to be great but frankly sucked.