Hypothetically, if I were a 3pp publisher...
I'd start with adventures, with a view to getting something out as close to release day as possible. In fact, I'd probably be writing it now, using the playtest rules.
Once the game is out, I'd take a close look at the provided rules modules, and come up with a list of areas that aren't yet covered. I'd probably cross the biggest few off my list (since WotC would probably be working on them already), and of course anything WotC have already announced. And then I'd look at the possibilities of supporting the others - especially any that are more old-school in nature.
I'd think very carefully about doing a setting, although I might drop some hints of one into my adventures, for later use (or not).
Incidentally... whether it's officially open or not, I'd actually be very surprised if it wasn't possible to support 5e using the OGL. After all, it was possible to do so with 4e, and everything we've seen so far seems to indicate that 5e is closer to 3e and/or the retroclones (which are OGL). And, where there are new mechanics, they seem to have deliberately favoured terms from 'natural' language, such as "advantage" and "challenge", which they can't really control. Though, of course, it would be nice if someone at WotC recognised this likelihood, and officially opened the game - much easier, and less risky, that way.
(And, since it really does need to be said: I am not a lawyer.)