Proliferating types is something I don't blame you for worrying about. Since your extension system is based on one extension per type, introducing a new type means extending all the necessary utilities again. At one point I thought that was fine, that I could just put in macros that performed several extensions at once. Now I prefer a base-extensions-on-support-for-other-extensible-things approach for Penknife, but I haven't had enough time to figure out what the pros and cons are.
I've definitely considered that path-search idea before, and I think at one point I talked about why I don't have much faith in it. The short of it is, if there are multiple coercion paths from one type to another, how do you determine which path to use? Be careful; if there's already A->B->C and I define a new type D with A->D and D->C, then code that expects A->B->C might use A->D->C and break. Technically you could propagate the transitive closure each time a coercion is defined, thereby creating the kind of stabiility needed to solve that, but I don't know if that's intuitive enough. Maybe it is. :)
It's definitely one of the goals of wart to minimize the number of methods you have to give a new type to get all the primitives to work. sort isn't generic because the comparison operators are.[1] I think python's __names__ give us a pretty good approximation of what we need.
