I'd totally disagree with this point. I think the whole thing wit Rorschach journal could easily open things for a sequel of sorts!
Are you saying that was the ending's intent or that-that's what could be intuited?
If it's the former, I disagree. I saw the ending as the superlative ending. It's basically Rorschach's revenge. He pissed in Veidt and Manhattan's coffee; the damage is done; we know there's gonna be a shit storm; the implied effect of the journal carries more literary weight than a full blown sequel that'll just feel tacked on.
