I am 60% confident its going to be Thierry because he is already senior VP of creative which is a major part of what they do; however, the fact they didn't announce the new leader does lead me to believe they are looking at outside talent.
I don't Thierry really wants to be UC president. If you remember Thierry's background has strongly been in set design and film....while the lands he makes are wonderful, his trouble is thinking out attractions....Out of his tenure, the best things he made were Spiderman (with Scott), the first edition of Space Mountain, and the creative direction of Diagon Alley. The last few projects he directly had hands in...he was behind a film camera and directing. I think that's way more his passion than anything else which means he would need someone to balance him. I think he knows his weaknesses and would probably like to keep his current position until a later time.
With the two upcoming projects being almost heavily physical based (EU and USB phase two), he would be great for set design, however when it comes to attractions he is too focused on his medium which based on results, people don't like as much. Then with USB phase two, not being IP based, only one attraction has he done that wasn't focused on an IP (Space Mountain in Paris).
I do think Scott Trowbridge may be trying for his way back to Universal as he is stuck solely focused on mainly Star Wars and the Portfolio Creative Executive position is probably extremely limiting on creativity and ideas and oversight compared to being head of UC in which you get a hand on how all projects go.