I think my ideal system would use something like role and power source from 4e. Leave the class part out of it and have role define what types of abilities you get and power source define your resource game.It's interesting, on the one hand I feel like I crave more flexibility, customization and more decision points. At the same time, I very much do not want a Skyrim-style system where everyone just starts as pretty much identical blank slates and develops their character entirely in response to what they do in game.
The idea of class, or at least something like it, putting some constraints and definition on who your character is and what kinds of things they are capable of right from character creation feels like an important part of playing an RPG -- especially a party-based one.
I think for me the ideal would be a system without fixed classes, but where you make initial decisions that significantly constrain your future choices within a very expansive combinatorial space of possible paths. Traditional classes would exist as templates, but you could tweak the template without having to rely on clunky multiclassing mechanics.
Since I mentioned Skyrim as an example of what I don't want, I think maybe the earlier TES games, which had predefined classes, but also the option to create a custom class, which affected how easily you could progress different dimensions of your character, are good examples of the kind of build mechanics I'd love to see in a future edition of D&D.