I like this idea. However, the next round of talents should not only cost much more credits, it may also be good to tie it to accomplishing a very hard quest or certain heroics boss. This would make it more interesting.
Better idea is to have it be unlocked at a high level, say 100, and you can only level it one level for each level you are over than 100. Then make the credit costs very high, and make the cooldowns equal to those of lvl 100-115 attack talents.