The only thing I'd change about TBN is the threshold for the MP refund. If the shield takes any damage at all, it should trigger. Being punished for having ilvl is one of the worst feelings.

If I really had to push for other changes, make TBN interact with other defenses. Before LD was changed I considered the possibility of letting LD consume TBN for an alternate (near) invulnerability skill that would essentially be a bigger TBN.