Columbia Website lists the following optional rules for Liberty (which are in addition to some optional rules in the actual rulebook:
LIBERTY OPTIONAL RULES
The following optional rules have been suggested by Liberty players. Gamers may choose to experiment with any or all of them in their games. Feedback is encouraged.
1) French Entry roll
At the end of:1776: Roll 11 or higher (enter in 1777)1777: Roll 8 or higher (enter in 1778)1778: Roll 5 or higher (enter in 1779)1779: Automatic (enter in 1780)
2) Prisoner Exchange Rule
A player can use a Supply card (2 or 3) to force a Prisoner Exchange (assuming both players have at least one unit captured). Both players can then choose one of their own captured pieces and return it to their force pool.
3) British West Indies Forces
British forces in the West Indies can not be moved or used until the French enter the war.
4) Extra Card Draw
Players draw one extra card each year (4 in 1775, and 6 each year thereafter). Each player then chooses one of those cards and returns it to the deck (not showing it to his opponent). Players then use the 5 cards in their hand (or the 3 in their hand in 1775) normally.
5) Indian Home Defense
Whenever a blue block ends a move in an Indian home town, the corresponding Indian block is immediately returned to its home town (if it is currently on the map and unengaged) and defends it (at B2).
6) Draw Result
If the game does not end due to the British gaining 30 or more points, or the British being reduced to less than 12 points, then the players get a draw if the British hold 25-29 points at the end of 1783.
Frankly, with the exception of No. 6, none of these are really optional in my opinion. The first two, in particular, are really necessary for regular play. If the French entry is delayed too long the game balance becomes badly skewed. And prisoner exchanges will almost never happen because one side or the other will almost always benefit from NOT exchanging.