Rita: I don't much like the idea of Brenda's bowing out, either under a cloud or under her own volition. I'd rather see a finale more like the one L&O:CI had, in which the two detectives just keep on keeping on. Things have changed slightly -- their relationship is closer, more personal. But they're both doing exactly what they want to do, and what could be a happier ending than that? If Brenda is going to be denied that kind of completeness...well, a bittersweet ending is probably the best to hope for.
Andrew: More of a tease than an actual cliffhanger. Holder, the secondary detective, tells Sarah Linden, the main detective, that a toll bridge photo shows their suspect to have been at a crucial place after denying having been there. That's the one final bit Linden needed to complete her case, so she makes the arrest. (I won't say who it is, in case you decide to watch the Seattle version.) Then the very last scene shows her on a plane with her son, ready to take off, when she gets a phone call from some civic archiving place. The caller says they can't provide her with the toll bridge photo she requested, because that camera was out of order for several weeks, including the date in question. No photos. Holder lied.
The obvious suspect (Belko/Vagn) was dismissed with a plausible explanation of his involvement, but in retrospect that might have been a red herring. However, there's this. We sometimes see TV shows in which a cop will falsify evidence to nail a criminal he knows is guilty, and Holder is trying very hard to make good on his first homicide case. The fact that Holder's evidence is nonexistent doesn't mean their suspect is innocent. They might have the right guy after all. What I find improbable is that Linden would go ahead with the arrest without actually seeing the photo herself. She's always had trouble trusting this particular partner, so why now? We're told all this will be cleared up early in the new series and a new case will then be introduced. So I guess Linden gets off that plane. No California wedding for her.