The Ori became the main villains. They were the evil version of ascended ancients. The Priors were their human minions who had been given a fraction of their powers.

That whole period of the show was blatantly an attempt to reset the story in an attempt to bring in more viewers. The SGC was almost completely restaffed with different characters. There was an attempt to even make a whole new SG-1 cast but it seemed pretty half hearted and kept rotating in the classic characters of Carter, Teal'C, and Daniel in varying combinations to assist Ben Browder's and Claudia Black's characters (I honestly forgot their names).

The idea of "sufficiently advanced technology looks like magic" was not exactly novel to the Ori. The entire concept of Gou'uld was that they presented themselves as gods. Ori seemed like writers being told they had to come up with a new villain capable of making the humans look like underdogs again, and so the power level of the villain went up.

If I recall correctly, the show was originally supposed to end when Atlantis was discovered. I suppose somebody got cold feet about canceling the flagship show, which led to the awkward situation of renewing it rather than fully handing the story to the new show.