Lots of things seem to be coming together for Marvel in May 2018. April sees the final four issues of the No Surrender storyline from Avengers. April sees Amazing Spider-Man #800. April sees Captain America #700.
April sees the release of the Avengers: Infinity War from Marvel Studios. And although they don't talk with each other anymore, Marvel Comics does try to have something compatibly named to run alongside the big movie event in the hope of skimming off some of that sweet movie money.
This is likely what the Infinity Countdown is counting down towards.
Bleeding Cool had previously reported rumours that Marvel Legacy and the across-the-board renumbering not-a-relaunch was in anticipation of an actual relaunch in 2018, which came with rumours of Ta-Nehisi Coates writing Captain America either from or after #700; Nick Spencer to write Amazing Spider-Man after #800; and Jason Aaron and Esad Ribic taking on Avengers.
Of course, all of this was before they got a new Editor-In-Chief….