Сertain versions of HHVM have revealed issues with data types in MediaWiki (e.g. T163646, T140878, T126871, T177134, T140864). Although PHP 7.0 is likely to hide those issues from view again as does PHP 5.6, I think using type hints in function declarations (PHP 7 supports type hints for scalar types) will make code more robust, will force repayment of technical debt (rather than using hacks like https://gerrit.wikimedia.org/r/381617, https://gerrit.wikimedia.org/r/302430 or https://gerrit.wikimedia.org/r/381616) and in the future, possibly, will lead to gains in performance (as does strict typing in HHVM when using repo authoritative mode).
I therefore suggest a goal more ambitious than merely dopping support for PHP 5.* (task T172165): making strict typing of function parametres and results compulsory in MediaWiki code.
This requirement can be enforced by automated testing, as far as I understand.