aramis erak
Legend
In the US, processes are EXPLICITLY not protected by copyright law. Explicit in the law itself, and pretty plain english.
Only the exact wording is protected by copyright. See US Copyright Circular 31 (PDF) - which uses a recipe as example. An RPG is functionally a recipe. Don't use the literal text, don't use the names of distinctive characters from the fluff, and they can't get you on copyright. See also Copyright and Games (webpage). Both links are to the US Copyright office.
There is another issue, tho - that of Trademark. Indicating 5E compatibility may run afoul of that. Trademark also (since the 90's) can be applied to "look and feel"... but that is a dicey situation. If you get a C&D, expect to go broke if you insist on going to court.
And a third issue, but one which isn't relevant - patent. Processes can be patented... but the window for patent on RPGs is long expired.
Only the exact wording is protected by copyright. See US Copyright Circular 31 (PDF) - which uses a recipe as example. An RPG is functionally a recipe. Don't use the literal text, don't use the names of distinctive characters from the fluff, and they can't get you on copyright. See also Copyright and Games (webpage). Both links are to the US Copyright office.
There is another issue, tho - that of Trademark. Indicating 5E compatibility may run afoul of that. Trademark also (since the 90's) can be applied to "look and feel"... but that is a dicey situation. If you get a C&D, expect to go broke if you insist on going to court.
And a third issue, but one which isn't relevant - patent. Processes can be patented... but the window for patent on RPGs is long expired.