First, some issues to bear in mind:
Bounded accuracy means that at high levels, it's theoretically okay to have access only to low-level spells. If your Burning Hands spell's requires a DC 13 Dexterity save in NEXT, well, you'll have a chance to affect most foes, because even high-level baddies only have a +6 Dex at most. Sure, if you have Meteor Swarm you'd do more damage, but your low-level spells aren't useless.
This is unlike in 3e, where at 15th level your top-tier spells might call for a DC 26 Reflex save, and your enemy might have a +13 or higher save bonus (I used a CR 15 red dragon as an example). That's fine for a dedicated 15th level wizard, but a dabbler who just has 1st level spells and a 13 Intelligence would be requiring DC 12 Reflex saves, which the enemies would pretty much always succeed.
So from the get-go, one of the hurdles to 3e-style piecemeal multiclassing has been avoided. Now you can pick up a few spells and they'll still be useful at high level. (It might be necessary to let their damage scale by character level, though, instead of class level.)
Second, feats in NEXT seem a lot more accepting of granting cool abilities than previously. Like, you can take a feat that lets you cast spells, which would have blown people's minds back in early 3e. If the game got a teensy bit more free-form, maybe it'd be possible to put all the things like fighter maneuvers and wizard spell slots and barbarian rages into the same pool. That might be going too far, but it's already possible to pseudo-dual-class yourself just by picking up feats that grant abilities normally reserved for a different archetype. Be an archer fighter, but pick up some healing magic through feats. Be a wizard, but learn a few fighter maneuvers so you can be a 'bladesinger.'
Personally, I'd prefer a more modular system, where you can easily pick from multiple options and toss them all into a single pot that is your character. As long as the underlying math of the game keeps the numbers in a narrow band, it'll be hard to become unbalanced, and the game will be a hell of a lot less complicated than some of the baroque multi- and dual-classing options of 1E and 2E. I'd also rather avoid Prestige Classes as things that restrict access to nifty powers. A better option, I feel, is to present a variety of slightly-above-the-curve feats, and put them in a section of "Setting Feats."
Say, "These feats typically should be tied to some organization in your setting, or require a quest to find either an instructor or some relic that can give a character access. Characters will have to work to achieve these impressive feats, which should make the players feel like they're being rewarded for investing in the game's setting. We suggest a few options for how characters might access these powers, but feel free to create your own backgrounds for them."
So the ability to, say, turn demons like you'd turn undead would require joining some organization that teaches that. And setting books could then create pseudo Prestige Classes by describing an organization, then listing the 4 or 5 feats members get access to. You still choose which feat(s) you want as you level, so not all Clergy of the Holy Nimbus have exactly the same powers. Indeed, by making the 'Prestige Class' requirements be legitimately role-playing based rather than stat-based prereqs, you'd let characters keep their main class and demonstrate their involvement in an organization by feat-choices.
Flexibility is, for me, the name of the game.
