When you look back through the whole series it might not have been the most dramatic ending or have the biggest twist but it was very satisfactory and very correct.
Right back in series 1 you have Buckells playing golf with Hunter and he allowed Dot to go in the back of the van to speak to Hunter with no-one else around. Jed has said he wanted it to be a character that was there from the start so it was never going to be someone like Thurwell.
I think the biggest point in it all is that the OCG don't need a mastermind criminal within the police force - look at Dot and Hilton. They need greedy and morally bankrupt people that are prepared to turn a blind eye, manipulate situations, give tip offs and be able to authorise corrupt officers working in specific departments.
It also leaves it very open to the fact that the likes of Carmichael and Osbourne can still be bent bastards but working from a higher place than the 4 H's who are essentially just caddies.
Ted went back and confessed to Carmichael not only because of integrity but for the benefit of the other 2 who he didn't want to burden with what he had told them. The ending shows they have bonded again after an episode of being honest with each other. It was Ted who had Arnott tested so that he could get help.
If they go another season I imagine it will be Chloe and Chris leading AC-12 with perhaps the main three working in the background to bring down Carmichael or Osbourne. I think it's probably done though.