i think itd be WAYYYYYYYYYYYYY to hard to make a nonlinear campaign.
you should make a SEMI-linear campaign (like in resistance, you can choose to betray the partisan, but eventually you end up fighting agaisnt the russians)
that plays out like a movie, so it feels like the campaign IS linear, BUT it isnt.
thatd be pretty cool, like if you made classic cop film cutscenes like the protagonist pulling off a daring hostage rescue in the intro hehehe. or a high-speed chase in which the protagonist saves the day.
thatd also be cool if you made it like a "lethal weapon" shoot em up action flick heheheh. or maybe some john woo special FX action movie, maybe like max payne (hmm, nah screw max payne ;D ) , maybe where you bust a drug ring or a mafia.