Yes, it would definitely have a very drastic development cost, I *personally* see it as worth it but it's a very subjective thing. Such a feature would obviously make any new content take longer to develop, but it would also add substantial replayability as you could experience the game differently in each form.
Perhaps it could be tested in a specific, isolated event zone initially, and if it is works out well there then perhaps it could be expanded to the rest of the game?