One of the things I like about David Allen's approach to his GTD product is that he seems to focus on developing his own unique discoveries to the maximum level, rather than being tempted to expand into other areas and sell add-ons in areas where he might not be the master.
I would say that GTD does not deal with all the areas you mentioned completely but one of the (perhaps) unique qualities of GTD is that it is capable of absorbing many other methods into it without breaking down. I think this demonstrates the real-world applicability of the approach as a hub for thinking and acting.
So, after implementing the GTD mechanics, there is endless potential to keep developing it, specially with regard to the six horizons and the Making It All Work book but, if you need an extra boost to deal with a particular issue, such as procrastination, you can read a book such as Neil Fiore's The Now Habit and implement it through GTD.
Fun is being on top of things - Sir Richard Branson