You're right that you can't reproduce the D&D 5e edition rules, Forgotten Realms maps, etcetera, because they are fully copyrighted and no open/free license exists for any of their content. You would need to get permission from Hasbro (not Wizards of the Coast), and Hasbro's lawyers wouldn't even give it a moment's consideration.
Making a “retroclone” instead would be legal, but also financial suicide
The only way you can “reproduce” D&D 5e rules is to follow the example of the retroclones: rebuilding the rules by altering the d20 SRD's text until it effectively explains the target game's rules in a new way.
However, doing this is a non-trivial amount of work — to do it effectively you have to recreate the rules from memory or via clean room reverse engineering, otherwise what you produce isn't a retroclone, it's a derivative work that's just plain violating copyright. Having attempted a retroclone, I can tell you that it's at least ten times more work than expected, and about three times harder to resist referring to the original when writing than you expect. And any hint of copying-then-editing (either literally, or mentally) left in the structure of your project would be legal doom.
In the end, the calculus isn't worth it for something like this when you intend to sell it: the amount of time spent rewriting the rules and then going over it with a fine-toothed comb to make sure none of it is going to open you to legal liability is so much that by the end, you'd have to charge hundreds for each copy to just break even in a reasonable amount of time.
Not to mention that, being WotC's current in-print edition of D&D 5e, you'd be nearly-instantly buried in Cease & Desist letters and maybe full lawsuits, regardless of how careful and within the law you actually are, and you probably can't afford the legal fees to prove it even if the project was completely above-board.