NS1 in-game Trailer Contest for fans and players to show what they got (with a good prize: three copies of NS2 or something ) a good trailer is probably only possible from a clan or group. One copy of NS2 can't be enough, or maybe something else as prize?

All contents will be gathered on forum, links will all be from youtube with specific "tag"s, read forum rules etc.



pros:
Ad for NS / NS2, getting the word out. (youtube)
(spreading NS = spreading NS2, if they like the game, they'll check out the website etc.)

cons:
The cost of prizes.
?

Many smaller teasers are more important, I think.
Short teasers to get them curious. (So what's this about?) Humour or shock-value works really well.
Then a trailer to let them know what it's all about, and get them really into it.

edit: though I hadn't really realised what the topic was about, because i'd only just skimmed the post, so this might be irrelevant.
As for the competition idea itself, a prize: something an NS fan would like - a free copy of NS, or a signed poster with a cory concept art. The competition idea isn't a bad one, really. It means that the developers wouldn't have to worry about making the trailers and such themselves, and can simply approve ones they find of a high enough quality and that really gets the idea of the game across.
Then again, only the devs have access to the actual game to take footage from it*; and they can ensure quality by producing it themselves, or hiring someone to do so.
*Of course, a solution would be to hand out stock footage to contestants for them to make a trailer from.