Here's an interview with Dominic West from the LA Times, which hints at the series finale. It also talks about how the condensed 10-episode season affected the storyline and a possible movie that "would have to be a prequel."
One of the more revealing answers:
"It’s an amazing feat of logistics how he has wrapped it up. Little story shoots that were sent out in Season 1 and you never heard of again come back and are sewn up –- I’m mixing hundreds of metaphors here –- but what you find at the end of "The Wire," and I think in life, is that all these characters that you’ve come to know and are fading away are replaced by a younger generation. So you see exactly who’s going to become the new McNulty and the new Omar, because the thing about the police work against drug gangs is they bring down one and a few years later they’re building a case against a whole new one. I suppose it's the idea that relentlessly things don’t change and you keep seeing the same thing again and again."