That series is what gave me the idea for it. I'm hoping not to step on any toes by doing it nearly exactly like the book though.
The gradual decrease doesn't have to be done on it's own. My idea was to have the pool get smaller. Say we have 500 forms for the Avatar to choose randomly from in the beginning. This is a big number but neither you nor your avatar are established as anything special so it's ok. Well, as you move on, move up, and determine what type of character you are, who you align with, what your proffessions are, each status will have it's own pool and hopefully the cross-sections from each pool will be substantial enough. Does that make sense?
You get to level 20 and decide you want to specialize in X tree of talents, well 50 forms for your Avatar no longer apply for 'in combat' because those forms probably wouldn't make sense fighting alongside an X talent.
As far as the gradual decrease in shape shifting, for the sake of making it easier, I suppose we could make it an accomplishment of some kind; an epic quest in end-game content "shows who you truly are" and then afterwards you get a certain title and your Avatar and you decide on a shape. Sort of cheesy, but do-able.