I certainly hope it isn't 5.5
That would imply a whole raft of niggling rules tweaks, and that's horrible.
I don't share that implication.
The ".5ing" I'm talking about is a substantial new product (or line of products) that often happens when sales of the original core rules begin to flag a few years into an edition cycle as the market saturates.
In 2e, that was Skills & Powers (or maybe the black-covered-book revision!). In 1e, Unearthed Arcana did something similar. In 3e, it got formalized with 3.5, with its niggling rules tweaks. In 4e, Essentials with appeals to "classic design" and drift away from some of 4e's organizational principles.
It's defined more by the fact that it is a reaction to existing edition design moreso than by any particular quality of the rules tweaks. A useful point of comparison is, say, 4e's Martial Power vs. 4e Essentials. Both new options, but one clearly thinking about the fundamental design of the edition, the other simply adding MORE STUFF.
I wouldn't be shocked if this included, for instance, a new ranger - you
could still use the old one, but now there's also a shiny new one that has a few years of feedback driving it! Maybe it gives you an option to add bonus spells to sorcerers!
Versus something like
SCAG, which is basically "more stuff!"
Could be wrong on that, though - if they just make a big book of more options, it'd be a ".5ing" in terms of business strategy only: a shot in the arm to a game a few years into its edition cycle.