Well...
spoilers:end of spoilers
A. The film does end with John setting out to rebuild the Corps, so there's that. I've seen some speculation online that we may get the likes of Simon Baz or Jessica Cruz as among the new recruits. And some of the original Lanterns could still be out there, or get resurrected. The GLC was rebuilt in the comics...in fact I think we could get a Green Lantern: Rebirth story down the line. But before that I want to see them play with this new status quo.
B. Agreed. I think the Rann-Thanagar War just wasn't needed. They should have kept the focus on the destruction of the GLC, Sinestro, Hal's corruption, and John's origin story.
C. Totally agree. The fights looked visually impressive but didn't stand up to logical scrutiny. A rookie John nearly defeating Parallax made next to no sense.
On a somewhat related note, there's been a persistent rumor that the DC Animated Original Movies have been building up to an animated COIE adaptation. What do you'll think about that? I do feel that with the Tomorrowverse they could potentially be building up to that (rather too rapidly I feel...Beware My Power has already leapfrogged over the formation of the Justice League, for instance).