From what the article says, and from his quotes, it seems that it'll be more likely an MMO, especially because he says "Wouldn't it be great if we could make a world that was always there for you?" RPGs have an ending, and it can't always be there for you, unless you start a new game, and technically that doesn't count as it always being there for you. Most likely an MMO, although I'm not too sure about Bungie making an MMO.
I'd like it to be an RPG, though. Like Owen said, there's not enough good RPGs.