I've been thinking about the following changes to make Tumbling a little less powerful.
If the defender is unable to make an attack of opportunity you use the standard tumbling rules.
If the defender is able to make an attack of opportunity he makes an attack roll at his highest attack bonus. This roll sets the DC for the tumble.
If the tumbler fails the roll by less than 5 he must stop in the first square that the defender threatens. He suffers no other penalties.
If the tumbler fails his roll by more than 5 he must stop in the first square that the defender threatens and the defender is allowed an attack of opportunity.
If the defender is unable to make an attack of opportunity you use the standard tumbling rules.
If the defender is able to make an attack of opportunity he makes an attack roll at his highest attack bonus. This roll sets the DC for the tumble.
If the tumbler fails the roll by less than 5 he must stop in the first square that the defender threatens. He suffers no other penalties.
If the tumbler fails his roll by more than 5 he must stop in the first square that the defender threatens and the defender is allowed an attack of opportunity.