The proposed changes are to add -Werror=implicit-function-declaration and -Werror=implicit-int for helping better prepare C code for future revisions to C and making the code more friendly with C++ programs. Additionally, for Fedora 26 on i686, the plan is to drop -mtune=atom. The tuning for Atom was done previously back when 32-bit Intel Atom CPUs were very common, but now less so, and many i686 Fedora packages are being installed on x86_64 and thus might run a bit faster on current hardware.
Those are the proposed CFLAGS/CXXFLAGS changes so far at least per this Fedora Wiki page. They are rather safe updates but not like the aggressive performance-minded compiler flag defaults used by Clear Linux.