WMF switched to igbinary, instead of php serialize(), to better use memcached memory and lower network I/O, but had to switch back due to HHVM incompatibility. Since we don't use HHVM anymore, we could consider switching back. The PECL client already supports seamless migration using the bit field flags.
It would really be useful for cutting down on bytes spent on things like WANCache wrappers, just from some quick use of var_dump().