TwoSix
"Diegetics", by L. Ron Gygax
Beyond the Wall is lightweight enough you can graft a lot of extra complexity to it if you feel its warranted. The only real downside to the BtW system, to my mind, is the retention of the old-school "5 weirdly named saves" system.I tried to blend Shadowdark/DCC/Beyond the Wall style mechanics into my OSE game, and found it was too much work for the result. So I instead would run those systems if I wanted that variant spell system, as they were effectively close to OSE anyway. I stuck with OSE/BX (Vancian) casting for my OSE games.