Tony Vargas
Legend
Sure, as soon as it stops sucking, or starts feeling like its sometime after 1989, for instance...Yes, you can homebrew entire systems - but at some point you're not actually playing D&D.
... but seriously, D&D at it's most authentic & genuine was also D&D at it's most varied from DM to DM, that's something 5e reaches for with its maxims of DM Empowerment, Rulings not Rules and being a starting point for us to make our best games.
That's literally d20, an OK open-source core system with a 20 year history, less than half that of D&D, of which it is, well, derivative. Ok, from which it was derived.The core resolution mechanic - roll a d20, add a number, and compare it to a target number - is what makes it D&D.
For the 26 years previous to d20, D&D used multiple resolution systems. Not just d20 to a DC and d20 under a target, you had d6 checks, several d% mechanics, and just whatever arbitrary dice (or other) mechanics the DM or author of a module came up with.