I'd say Doubles or UU would be the best as they're still similar to OU, but different enough to be a nice break from the standard battling (on top of the fact that people will actually have at least some pokes to start out with.) I'd be more in favor of doubles simply because of the enourmous variety of pokes that end up having niche viability alongside standard OU threats.
As many of you know, I was pretty solidly against NFE as our first non-OU tournament because I just don't feel that one week is enough time to not just breed/buy a team, but to learn an entire new tier and assemble a team that works in the first place, alongside the fact that NFEs are entirely tier specific (you can use UU pokes in OU, you can't use NFEs anywhere but NFE or maybe PU). Once the tier is more established, however, and everyone starts actually having teams prepared for it, I wouldn't mind having another NFE tourney (but I think it should be a much slower transition from standard battles.) Same goes to LC to a lesser extent (since there's actually a way to test it without showing everyone in the tourney your team.)