I wonder if there might be some appeal in balancing AW/E/D around increased flexibility. Something like, every spell has 3 versions, one for each level of frequency. You have x spell slots, but you can choose to memorize any combination of spells. You could memorize x at-wills, x encounters, x dailies, or some combination thereof. 3 options might be overkill, but even if you dropped it to just at-wills and dailies I could still see it working.
For example, Invisibility. The at-will version might only last until your next turn (1 round), while the daily version might last for an entire encounter (5 minutes). If there's an encounter version, it would be somewhere between those options. That's just an example of course; the standard could instead be that at wills last for rounds, encounters last for minutes, and dailies last for hours. Either way, those are just fiddly details.
So, if you had 5 spell slots, you could memorize at-will Invisibility and 4 other spells, or Daily Invisibility five times, or some combination thereof. It might be a reasonable compromise between the various editions, and it would allow players to play their casters as they prefer. Like the 3e warlock, memorize at will versions of spells. Like the 3e wizard, memorize dailies exclusively. Like 4e casters, take your preferred combination of frequencies.