Roll each round. D4 = ranged, d8 = melee, d12 = spell, d6 = anything else, +d8 to swap gear, +d8 for bonus action, low goes 1st #wotcstaff https://t.co/7nSwTNfHCI— Mike Mearls (@mikemearls) May 16, 2017
This gave me the simulations stages of AD&D, the element of planning of side-based initiative, and the delusionally interesting bits of 2nd Edition AD&D.
The first things I'd adjust would adding something beneficial for charges vs normal "action + move", tie spell casting time in (AD&D casting time + d6 or something, or better yet use the Mythus classifications: http://vaultsofnagoh.blogspot.com/2013/02/lets-read-mythus-pt-23.html), and give some benefit for defensive reach. But the core idea above is so strong, I don't want to mess with it too much.