By 'Big' video are we talking about something similar to Get 2? (in both quality and length)
If so 40mins is more than acceptable
I think music needs to mixed up all the way through (like MTB films) though i agree there should be no dubstep
Hip hop, chill/liquid d+b, funky breaks (A-skillz/Krafty kuts), acoustic and rock all have attributes which can enhance a videos watchability massivly. But done wrong can really make a segment hard to watch.
I think individual rider segments should show said rider in a variety of locations
Group rides are a cool way of showing off the social side of trials and this is where the mixing of riding styles should be used (24", streety, compy, TGS etc)