We already have subnamespaces in std (e.g. std::chrono), so your standardization plan could be realized as s/boost/std.
It could if I wanted to propose std::mp::if_, but I don't. I want to propose std::mp_if.
I know that this makes the library more difficult to use in other Boost libraries where there's no convenient place to put the using directive. :-/
To expand on this a bit: There are, in general, two main modes of use of Mp11, serving two audiences. One is the "easy mode", where one includes <boost/mp11.hpp>, combines that with `using namespace boost::mp11;`, then goes ahead using mp_this and mp_that without qualification. This serves (a) people who play with metaprogramming in short test cases, (b) people who have a need for a metaprogram in a .cpp file (or an internal header file not meant for public consumption), whether library or application one. Mode two is in use in header-only libraries. There the library author is generally reluctant to employ the using directive, which forces the comparatively awkward mp11::mp_this style. I realize that my choice gives preference to case one at the expense of case two.