It's a thought experiment
It'd look a bit like Essentials, really - heck, 5e /is/ built on a lot of d20/3e/4e mechanics, anyway, it's older-ed pedigree is mostly attitude & feel. It's a lot easier to extract balance from a system built with it from the ground up than to add it in after the fact, and, ultimately, that was the main sticking point in terms of 'feel' between 4e and other editions.
So, sure, first take dailies away from martial classes, expand the power of the Wizard, in particular, and other casters in general, like Essentials did. And just keep going from there. Take at-wills away from the martials, too. Isolate encounter powers in the Warlock, one sub-class of the fighter, and an odd feature here or there. Radically expand dailies. Start by keeping a daily use of each daily spell you ever choose, rather than training them out, expand that from up to 3 or 4 dailies, total, per day, to 3 or 4 dailies per 'spell level' and one or two for the higher levels. LFQW re-established. Well on your way.
Slicing the last 10 levels off the game and dialing 1/2 level down to +4 over 20 levels is almost cosmetic, at that point.