We already kind of have a split when it comes to d&d. This would be attempt to recapture players that play older editions and put them under one edition. Basic is core, advanced is modularity that was promised in 5e and never delivered.
Splitting the lines is best compromise for a game that wants to capture widest possible audience and be one game that does it all, like D&D tries to do.
How I would do a 6e Basic (although many of these changes impact the 6e engine as a whole):
-Stats are as previous editions. Mods are stat-10, instead of (stat-10)/2. I'm agnostic on how stats are generated.
-There is only one kind of check in the game (an ability check). d20+mod >= Defense (either one of target's stats or environment value). No distinction between attacks, saves, ability checks (major pain point for novices.)
-Proficiency bonus gone. No skills, all checks are ability checks.
-Background, ancestry, class features can provide advantage to some ability checks.
-Advantage can stack, be removed. Encourage players to seek bonuses/avoid penalties without "sorry, rules say you can't have 2 advantages". 2 advantages + 1 disadvantage = 1 advantage.
-Classic and popular ancestries in the book, provide one active ability, and one or two passive abilities.
-No stat bonuses from ancestry or backgrounds or anything else. Causes optimizations problems unneeded for Basic.
-6 classes in Basic. Fighter, rogue, barbarian, mage (blaster), healer (defense and buffs), and psychic (illusion/mind magic/trickery). New class names for magic classes so previous players don't bring in legacy concepts and expectations. The familiar casters are in the Advanced book.
-Classes have limited options. Casters gain magic powers as class features, not as selectable spell list. No subclasses. No feats. Some levels have selectable features depending on class (generally between 2-4 options.) ASIs are every even level, giving +1 to main stat and +1 to another stat.
-All classes are short rest classes. 2 short rests per day, recharged on long rest. Short rest returns class resources and half HP.
-Long rest (end of day) recharges short rests and all HP.