I don't think 21st CB is a rock opera, and Green Day have never said it is either. It has characters and ambitous music but it's not a rock opera. American Idiot's an opera because it tells a story, it has a clear storyline with a beginning, middle and end that you can follow from start to finish. 21st CB just makes use of characters to express ideas and feelings, it's not the same thing.
I think 21st CB is a very personal album, the fact that it's expressed using characters names instead of saying "I" or "me" doesn't make much difference to me. I find it totally different from American Idiot. The next album could be totally different whether they go down the road of using characters or they don't, and whether they make big ambitous music or simpler music. There's so many other variables it could be a progression either way, and brilliant either way.