I like the idea of the Thief-Acrobat. However, I'm sort of stuck on the idea of making it a class unto itself rather than a "split class specialist" (I believe that is the term used in UA.) Last night I was tinkering with the class, and wrote it up as a level 1 to level 20 class unto its own right rather than starting at level 6 and progressing to level 23.
I was going to post it, but then I took a second look at the class and thought, "Is this something that someone would really want to play?" As written, the thief-acrobat is a thief who has traded some abilities for others. However, as a stand alone class, the acrobat is just a guy who can propel himself through the air in various ways. Ok, he has a few abilities beyond that, but you get the idea.
Perhaps I will post the class as I have written it up, but from where I'm sitting now, it just doesn't seem like something that stands on its own very well.