I voted 5.3 though I think it's closer to 5.2. Nothing printed in the 50th ani edition will contradict anything that has come before. Something like the below I think could work, as it wouldn't make purchase of the set mandatory for those who have older books, but would make a more robust Core 3 for new players and DMs.
PHB - Addition of select classes, subclasses, races and spells from other printed sources, but nothing removed and nothing brand new
MM - Addition of more robust humanoid section, possibly another dragon type, possibly additional monster variant sidebars
DMG - Addition of variant rules for Classes (like the UA), addition of something akin to the Heroic Chronicle from EGtW, addition of variant rules for social/exploration pillars