Now that Favored Souls have been out long enough that we've all at least seen what Leap of Faith is like, it is clear that the Acrobat specialty is going to have to get a similar feature if it is to be worthy of the name. The natural place for it would be tier 3.
The Abundant Step and Leap of Faith of the Monk and FVS classes are the premiere acrobatic movement abilities in DDO, and no character without something similar can provide nearly that level of ability at reaching inaccessible places, which is thematically the focus of a Thief-Acrobat.
Because Rogues don't have an expendable resource like Ki or Spellpoints, another kind of penalty must be added to prevent spamming the ability. I suggest doubling the cooldown from what FVS has, and also debuffing yourself from the strain, causing a -2 penalty to Strength, Dexterity, and skills for 10 seconds.