Luck is a major factor in Monopoly, so to be fair, a new player would have to be able to have a 50% chance of surviving one lap of the board. I would take the value of all claimed land and give the player 50% of that value. The amount taken would be a progressive tax, but one that would not change the current rank of all player's monies and land value.
If all land has been claimed when a new player enters, I don't feel a new player could succeed. It would take a much larger tax to all players, so the new player could selectively barter and purchase land from another player, but to win, other players can hold land and try to siphon the new influx of cash from the new player.
An interesting experiment, I like it! I believe the game German Railways (a "cube rail" game) is similar where players get unequal turns.
That gives me an idea, how about an ongoing game of Monopoly where players can join at any time and invest in bigger (or smaller) players?
edit: to answer the question more directly, I'm fairly certain it will never be "fair". But how exactly are we defining fair, here?