I think it would be nice if we could choose mastery of a different type than the current ones.
Instead of donating account bound abilities they could contribute to the growth of the account of several other ways.
For example, it might be interesting to have a repeatable mastery that transforms all the experience gained into some sort of usable material or liquid money (similar to pvp / reward track on level up).
Mastery of this kind could make the post-level 80 progression more interesting instead of becoming just an acquisition of spirit shard.
I believe that gw2 can become more coherent if the experience becomes more and more an interesting currency.
Do you think a more advanced mastery system with more choices could be interesting?