Facing an unepected move in opening

In general, the best way to deal with any unexpected move in the opening is to simply fall back on the four opening guidelines: place a pawn or two in the center, castle, connect the Rooks, and then aim the Rooks. If you do those things you are guaranteed to get a respectable opening, even if it is not the absolute most efficient way of dealing with the enemy moves.

In addition, if the enemy planning involves a fianchetto then you will probably want to build up a pawn chain on the same color as the fianchetto Bishop