Well, I just realised, I've never seen the words linear or nonlinear used to discribe the plot or writing of a game, only the gameplay. I don't know how a nonlinear plot works, only gameplay, and I was thinking only in mixed terms: linear and branched storylines, and linear and sandbox gameplay.
I really have no idea what other solution to game writing there can be, other than linear and branched storylines, although I may be using a broad definition of 'branching.'
The 6th way designers communicate? With their fists.