I see no reason for it to fuse given multiarch works now. I expect
people to have amd64/x32 multiarch systems though since the majority
of programs don't need a 64bit memory space so you would gain smaller
binaries and less cache usage due to the smaller pointers. The kernel
would obviously be 64bit. For those few programs that have a use for
64bit memory space, yo would use amd64 packages.
Essentially I think it makes sense to run 64bit kernel, with x32 as the
default and amd64 for select applications, and perhaps i386 for the few
things that can't be bothered to upgrade away from old 32bit.