It's very simple to produce content for 5E under the existing OGL. Follow these rules (IANAL but this is how all the smart 3PP are doing it so far
1. stick to the OGL as given, so adhere to its stipulations and requirements rigorously.
2. Do not duplicate stat block formats or other visuals that could lead to your product being mistaken for official D&D 5E content. Do not use any material you can attribute to open content or original content.
3. You are deriving mechanical equivalence from the 3rd edition OGL product, so do not reference 5E official products, and for the love of all gods don't label your product D&D compatible; it's not....it's compatible with the 5th edition of everyone's favorite fantasy game, nod nod wink wink. Strictly speaking this is very conformist with the OGL because you are not duplicating the exact mechanics of the source (3.5 rules). Mechanical equivalency with 5E is not forbidden in the OGL at all. It's how the retroclones were legally produced; the OGL was specifically written to allow 3PP to create derivative works founded on 3.5, and mechanical equivalency with 5E seems to satisfy that criteria. Odds are any OGL that might be considered by WotC will not be as permissive as the existing OGL, for the record.
4. Do not duplicate 5E text. Only duplicate identified useable OGL open content text if you absolutely must.
5. Rigorously adhere to the OGL IP exceptions. No mind flayers in a 5E compatible product because they are protected IP in the OGL 1.0a, for example.
6. If you convert or derive material from OGL sources make sure you attribute them correctly.
7. If you're nervous, look for name-equivalency when necessary; i.e. negative energy vs. necrotic. I think you don't need to worry that much, though.
8. Do not stick D&D anywhere on your product. (Second warning). 5E or 5th edition compatible is fine, apparently. Or if you're really nervous just label it "compatible with the very latest edition of the game as of 2015" or something.
9. Make sure you identify and protect your own IP in the OGL, but feel free to identify anything that is open content you want, or which derives from other open content.
10. As a reminder make sure your use of art is legal (i.e. you paid for it and attribute it correctly, and most likely make sure it's not designated as open content unless you and the artist intend for it to be).
....So stick to the OGL as written and you can already do this. It's just like writing for OSR retroclones, and you don't even need to emulate the rules, just the numerics and mechanical equivalency.