alot of people say they like long missions, but with a campaign you'll end up using all your ideas in one shot with a long level, so for you id say make the mission short, and action packed. Also make it challenging. If its the first mission at least put in some kind of cutscene. Voice work I could help you with. I can record my voice and make lip files. I still don't know how to do sophisticated camera scripting, well I haven't tried and i'm too lazy is the thing. But quite simply write down an idea list, don't use up all your ideas, make each level lead into the next somehow, voicework is good, basic camera scripting using the trigger, don't make the mission too hard.