I personally think this is the best theme yet. It's a video competition, it's not supposed to be easy.
There are all kinds of effects you could use. A nighttime scene would be great and it's really easy. Take a clip, duplicated it, match them, and adjust the levels on both. Make the bottom layer dark, make the top layer bright, and adjust the top layer to fit. It's not that simple, but if you make brights brighter, and darks darker, and you adjust the layer properly, I'm sure that would look good...
BTW, I've never tried that, but instantly that would be the first place I would start. If you want it to look production quality, there is going to have to be a lot more to I though. I've done simple scenes, such as the IGTC 2009 Intro with the My3id car, that have been over 6 layers.
Now keep in mind, nighttime is not weather... Most people are going to do a rain effect, but keep in mind, it doesn't have to be raining. For instance, the ground can simply be wet.
Lynce did a video trying to create that effect. I don't know how he did it, but I know exactly how I would do it and it's really quite simply IMO. I'm not going to drop the bomb on this one, because it is more technical than doing a night scene and I don't want to spoil it for the competitors.
Just keep in mind guys, this event is for editors of all skill levels. Making something production quality, such as Lynce's work, should not be the goal for all of you. If you guys saw my last video on here, you might know what I mean. That is not production quality, but it looks decent. That would be the mind set I would have when preparing for this contest.