"Four - It would be a hardcore adventure game driven by what made that era so great. No tutorials or hint systems or pansy-assed puzzles or catering to the mass-market or modernizing. It would be an adventure game for the hardcore. You're going to get stuck. You're going to be frustrated. Some puzzles will be hard, but all the puzzles will be fair."

Difficult, in the age of the ubiquitous walk-through. You'd need to introduce randomization so that each instance of the game was unique, but even then, sites can describe methods of finding the solution.