Could you please make the Acrobatics skill an opposed check for a person trying to move through threatened squares and through an enemy's space? A fixed DC is really not all that great.
The reason:
With a fixed DC, a person can use acrobatics to avoid the attack of opportunity from a 20th level fighter exactly the same as a 1st level wizard (or from a dragon just as easily as a simple goblin). Sure, the consequences of failure may be different, but a simple goblin should be easier to tumble past than a dragon.
By having a fixed DC, people get to the point of auto-success regardless of the creature he/she is trying to tumble past.
The solution:
A solution that a DM of mine uses (and I now use) is to make the acrobatics check an opposed roll versus the creature's base attack bonus. This makes the dragon tougher than the simple goblin. If you have a better solution, by all means, but the check should be opposed in some way rather than a fixed DC.
Thank you.