I agree with @mdboom and @leejjoon that that location implies a tie to the `WCS` object rather than to a future "generalized" WCS.  The question is where the gWCS ends up - if we end up creating `astropy.gwcs` or something, it's a problem that `WCSAxes` is in `wcs`.  But @astrofrog is right that we may want to end up putting the generalized WCS in `astropy.wcs` anyway.  

So if we are *sure* we don't want the future gWCS stuff in `wcs`, than it makes sense for `WCSAxes` to be elsewhere.  But if there's a chance of it being in `wcs`, I agree with @astrofrog that it's more expedient and understadable to put it here for now.