Any particular reason to use 2.0 instead of 4.0?Tier 1: Creative Commons Attribution 2.0 Generic (CC-BY 2.0)
So, free/open source licenses can generally be broken down as either copyleft or permissive. Copyleft requires that you reshare your modified code, while permissive lets you do pretty much whatever you want, including not resharing code. Copyleft is further broken down into a weak form and a strong form. The weak form says you only have to reshare the parts that you modified, while the strong form says that if you use any original copyleft content, the entirety of your code has to be reshared under that license.
There are use cases for both copyleft and permissive.
Permissive often works better for libraries that others will make use of, but are unlikely to modify (or if they do modify it, it's likely to be very specific to their own program, and not as generally useful to share), or where it's more important to support people using well-made/vetted/secure/performant/etc code than worrying about how they are using that code (such as in proprietary programs). Strong copyleft works well for utilities where you want to ensure that modifications and improvements continue to propagate throughout the coding community. Weak copyleft is often more appropriate for entire programs that have lots of intermixed parts.
CC-BY on the core mechanics would be analogous to a permissive license on a library. Most people using the mechanics are unlikely to change them in any meaningful way, so you can just plug them into your own use case and go. Those who do modify them (eg: Level Up tweaks things such as changing exhaustion to fatigue/strife) would likely be encouraged to reshare those modifications in the same CC-BY fashion.
Because of the nature of TTRPG systems, a strong copyleft license is kind of a non-starter for most publishers. Even aside from "product identity", there are likely cases where the publisher simply doesn't have the right to share some content (eg: an artist that licenses art to the publisher may not grant resharing permission).
Which leaves the weak copyleft as the remaining form to look at. However format becomes a stumbling block when comparing code and a game product. A weak copyleft on code can isolate changes on a per-file basis, such that you only have to reshare specific files. There's no such delineation in a print product. With print, it's the specific elements and type of content being shared that matters.
Of course those are just broad categories. There's lots of finer detail that show up in individual licenses. The Ms-PL (Microsoft) includes clauses on patents, and explicitly excludes a license on names, logos, and trademarks. The MPL 2.0 (Mozilla) says you have to reshare code that you modify, but if you don't modify the code you aren't obligated in any way. It easily combines with other licenses.
Overall, I'd posit you'd want a primarily MPL-style license:
- Basic sharing provision: Functional changes to the licensed content (ie: changes that result in actual differences in the outcome of the game, as opposed to formatting, layout, arrangement, etc) must be reshared under the same license. If no functional changes were made, no resharing obligation is created.
- Applies to: Core mechanics, and the SRD (a sufficient implementation of the core mechanics to run the system).
- No license is given for the use of product names, logos, or trademarks, or any suggestion of endorsement by the licensor, except where explicitly granted (for example, a logo that is approved for use on character sheets). Notice of compatibility (there's a term for it that I can't remember) is fair use, though.
- There is no restriction on the manner or medium in which the content is used.
- This license is compatible with using a CC license on any content above and beyond the core mechanics and SRD (eg: lore, maps, characters, etc). This is in no way required, but should not conflict with any other license provision if the publisher wishes to do so. For example, the publisher might use art which is provided under a CC license.
Beyond that, you may have a game system license which includes the entire contents of the product. This license may have additional restrictions on its usage. There may be variants on this for specific use cases (eg: video games, VTTs, selling in the app store, etc).
So basically, two license tiers, rather than four. Additional contracts with other entities (eg: app store, DMs Guild, etc) are an entirely separate matter, and not part of this license structure.