I don't think it's enough to rewrite but use the same vocabulary: you have to establish a substantially different vocabulary OR establish a distinct OGL lineage for the vocabulary that predates 5e.
Heh. I can take the entire text for Frank Herbert's Dune, change all the proper names, rewrite every sentence (terribly) in my own voice, and get my butt sued off for publishing a derivative work without license from the copyright owner.
If you use the terms "ideals/flaws/bonds", if you use the same basic page layout format... basically, if you can glance at it, and wonder if it came from WotC's book, it may be copyright infringing.
I am not an IP lawyer, and certainly not a US IP lawyer. So I'm just working from the statutory text here.
17 U.S. Code § 102(a) states that:
Copyright protection subsists, in accordance with this title, in original works of authorship . . . include[ing] the following categories:
(1) literary works;
. . .
(5) pictorial, graphic, and sculptural works . . .
17 U.S. Code § 106 states that, subject to certain exceptions which I think can be mostly ignored for present purposes, the rights of a copy-right holder include:
[E]xclusive rights to do and to authorize any of the following:
(1) to reproduce the copyrighted work in copies or phonorecords;
(2) to prepare derivative works based upon the copyrighted work . . .
17 U.S. Code § 101 defines "derivative works" to include works
based upon one or more preexisting works, such as a translation, musical arrangement, dramatization, fictionalization, motion picture version, sound recording, art reproduction, abridgment, condensation, or any other form in which a work may be recast, transformed, or adapted. A work consisting of editorial revisions, annotations, elaborations, or other modifications which, as a whole, represent an original work of authorship, is a “derivative work”.
The challenge in producing any clone (retro- or otherwise) is to avoid infringing WotC's rights either in respect of its copyrighted works, or in relation to derivative works based upon its copyrighted works. The relevant modes of derivation, when it comes to RPGs, would probably be "abridgment, condensation, or other forms of recasting, transformation or adaptation".
Whether or not it's legal, I think OSRIC is sailing rather close to the wind. A relevant consideration is the fact that WotC has released the 3E SRD (which OSRIC cites in its own s 15 declaration) as OGC under the OGL. And the OGL licenses the use of that OGC, and defines the OGC as including "the game mechanic and includes the methods, procedures, processes and routines . . . and means any work covered by this License, including translations and derivative works under copyright law".
The mention of "methods, procedures and processes" in the OGL is an allusion to 17 U.S. Code § 102(b), which states the following limitation on the scope of copyright:
In no case does copyright protection for an original work of authorship extend to any idea, procedure, process, system, method of operation, concept, principle, or discovery, regardless of the form in which it is described, explained, illustrated, or embodied in such work.
My way of framing the question about OSRIC and other clones is, therefore, in two parts:
(1) To what extent are the rules of an RPG, such that a gnome has a 50% chance of detecting sloping passages, or that a fighter has d8 hp per level, "ideas, procedures, processes, systems, methods or principles"? As opposed to, say, elements of a story? The answer to this is not uncontroversial among RPGers as a matter of playstyle (contrast "sim" players with "gamist" players), and the legal answer isn't obvious to me. (But might be obvious to a good IP lawyer.)
(2) If a work (like, say, a clone rulebook) is, in actuality and intention, derivative of unlicensed works (eg AD&D, B/X, 5th ed Basic, etc) but is capable of being formally characterised as derivative of the 3E and/or 3.5 SRD, has it been licensed by the OGL? I don't know the answer to this question either - it seems to turn both on the proper construction of the statute plus the proper construction of the OGL as a contract, and I'm not strong enough on either point to properly speculate.
