Well, the way I look at it. If it's going to be an event ran by the community. I suggest the event itself should be something to bring the community together, such as the emergence of a bigger enemy, like the flood among the UNSC and Separatist Elites. I'm thinking, something like a Vong/Reaper sort of threat. A massive threat where everyone could get involved. You'd need the fighters and the ambassadors trying to keep the strings of allegiance between each group from snapping and leading to ruin. I'd like something like this, and I've already got 3-4 ideas for a enemy species rolling around in my head. Something that can unite the community in more than a want to have fun. But also to beat back something nobody has ever seen before, fighting for their survival.
I'd say we should try to make these creatures as alien as possible. Because frankly I think chemical based warfare would be interesting, a high enough level of understanding of science to be able to recreate force like abilities and possibly outdo them.Which, frankly I think the idea of there being something "man" made that could withstand and possibly surpass the powers of the force is an interesting concept that would make most force users squeal in terror.
However, something like the flood would also be interesting to combat. Especially if we could have a gravemind. Anyway;
thats my thoughts on the matter.