I have seen various attempts at new phases. Some want them for the 6th player, and others want it for a phase instead of Prospector for the 5th player.
All attempts that I've see have involved direct player interaction, which we all know is a Bad Thing. It is definitely tricky. Here are some ideas you can try; I just made them up, so they are untested. Flesh out as you will.
1) Prince (requires 2 prince markers) - No function. Privilege: Take one prince marker, and place the other marker on one of the following: a good, a space in the trading house, a space on a boat, or a building. The object marked is reserved for your usage.
2) Unionist (requires a union marker) - No function. Privilege: Place the union marker on another phase. That phase acts as prospector, instead of its normal function. The union marker is removed after the phase is chosen. The union marker cannot be placed on the Prospector.
3) Overseer - No function. Privilege: Take a VP
4) Developer - Each player may return one building to the supply. S/he receives the GP value of the building. Privilege: The developer receives an additional GP.
5) Wholesalers - Each player may buy up to three goods from the supply. Corn costs 1/2 GP, rounded up. Privilege: take an additional good.
6) Hotelier - Each player may turn over one of his or her unoccupied plantations and receive 2 GP. The spaces may no longer be used. Privilege: receive 4 GP
7) Soldier (requires some privilege chips) - No function. Privilege: Take a privilege chip. Play the privilege chip during any other phase. Receive the privilege for that phase. Each privilege chip may be used only once per phase, and may not be used if you have already received the privilege for that phase.
8) Forester (requires wood chips) - No function. Privilege: take a wood chip. You may trade a wood chip in to pay 1 less for a building (max 1 per building), or you may trade a wood chip in to add an additional good onto a full boat (max 1 per boat).
9) General - Each player may place a colonist onto any boat for 1 VP. Continue around until all boats are filled, or all player's pass. A player may still ship after passing once. You do not get Harbor bonuses for shipping colonists. Privilege: +1 VP
10) Beureacrat - Repeat any other phase taken this round.
If you have 5 good players who really want to spice things up, replace one of the Prospectors with the Governor Card.
The Governor is now a choosable role. When chosen, nothing happens immediately, but that player will start the next round. If the Governor role is not chosen, a doubloon bribe is placed on it as expected, and play continues as per the regular rules...whoever went second in the current round starts the next one.
Predicting game flow gets *much* harder with this variant, which is my group has become quite fond of it.