We're not sure if the Ori are dead, and even if they are, there is still an entire Galaxy of Fanatics who have been riled up about killing the infidels.
Adding to the fact that Adria has also ascended and who knows what trouble she will cause.... Look at what a partially ascended Anubis was able to do!!
The two following films are not related. The Ark of truth will be bringing a complete end to the Ori story line, that means dealing with those followers who are still lurking out in space.
The second film, Continuum I think it's called, is about a shift in Time, where only SG1 seem to remember the original time line. In the new time line, Baal and Adria, the Gould that used to reside in Vala are about to invade and conquer Earth. The producers have stated that while SG1 will fix the time line, there will be some fall out, so not everything will be the same.