assuming it doesn't pose any major legal problems, a piracy board seems like the most likely to succeed of the non-gimmick boards. images only seems like the most likely gimmick to succeed or to play out in interesting ways and is the one i'd vote for.
most discussion boards could really do with a "business plan", some sort of idea of who they're aimed at and why existing boards are insufficient. especially to target the question of "is this a board for existing users to talk about a topic, or a board to bring in people who want to talk about that topic?" - if the former, making a case that it can't really be discussed on existing boards, and if the latter, suggesting how people can be brought in.
although /roulette/ is fine when it's just casually throwing up topics, there's a longer term risk of having a large number of neglected boards be nodded through because 2-3 people were using them.
although i'm not against /roulette/, it might be useful to look at ways to vitalize existing smaller boards. perhaps by making people off-site aware they exist, trying to attract (say) artists to /draw/ even if they're only left-leaning, not particularly interested in posting on /leftypol/ proper.
(though that raises a longer-term question over the site's aims: should /leftypol/ always be a politics website, or is it a general discussion website that leans left and has a left-wing politics board?)i'd be remiss not to mention an idea that may or may not be mad: create a new board if someone (or a group) volunteers to moderate/administer it, then assign them the responsibility for getting it up to speed within a trial period, ideally with quantifiable targets of success/failure agreed in advance, perhaps with intermittent review. i can see how the targets might be gamed, or how a decision to terminate the board might lead to drama, or how a board mod could go mad or whatever, but it might open up some innovative approaches for running a small board without much extra work being needed from the existing mod team. (assuming this is possible with existing software)