This is about removing cantrips (at will magic) and fixing the spell-equivalency system.
Really, it seems as much about re-magicking as de-magicking. That is, magic is ubiquitous (through a day) via cantrips and (across classes) through spellcasting (which every class has) and spell-equivalency (which the remaining sub-classes are all designed around to some extent). Ubiquitous magic doesn't feel special anymore - familiarity breeds contempt and all that - you lose that senseofwonda that's emblematic to the broader fantasy genre.
Do I understand the problem?
In short, this post comes closest to what I've been thinking:
1. Remove cantrips.
Check.
3. Wizards, Clerics, Druids remain relatively unchanged (other than loss of cantrips).
So we're not 'replacing' cantrips with Extra Attack, weapon proficiencies, or a few more spell slots or scaling spell damage or anything?
2. Fighters, Monks, Barbarians, Rogues are restricted to non-spellcasting subclasses. Any incidental uses of spells or cantrips are removed or re-written as class abilities.
Re-writing something as a class ability still leaves it a spell-equivalent. And Ki, Rage, CS dice/maneuvers, Action Surge & Second Wind are surely all calibrated as spell-equivalents, as well.
It seems to me that once you've re-established the defining sense of magic as a limited (within a given day), resource with correspondingly greater power/versatility/importance,
the only way to truly de-magic something else is to make it unlimited on the same time scale, and of correspondingly lesser power/versatility/importance.
In another thread I pointed out the early-Next-playtest MDD mechanic. As it's a limited resource only within the timescale of a given round - essentially 'at will' compared to real magic - it could be a way of adding a little management/interest/differentiation among fighter, rogue & monk sub-classes.
4. Sorcerer (to a lesser extent) and Warlock (to a greater extent) ... I don't know. Not sure how easy that they would be.
The Warlock has more of it's DPR-balance in it's amped-up Eldritch Blast, and it is on a short-rest schedule, which is also a more-available, thus less-special take on magic, so just doing away with it might be an acceptable answer. The Warlock concept, could be folded into the Sorcerer. Sorcerers from 1st level on have innate magic, McSorcerers who gain power later must get it from a patron?
5. Paladins, Bards, and Rangers re-done to remove casting. For obvious reasons, as you point out, bard would be tough.
The Ranger's already gone spell-less twice, now, though the 5e attempt didn't go over well, and was still spell-equivalancey. The Bard could lose spells and spell-equivalent bardic inspiration, and bring back Bardic Music (which needn't be literal, might even be (pi) literary) like in 3e & earlier, including inspire
__ (courage, competence, defiance, etc) as more or less at-will (maybe requiring concentration or something?) bonus actions it could do whilst fighting or otherwise contributing, and maybe counter-harmony to dispel charms & magic and the like (or suppress it while the music continues?).
Actually, that sounds like it'd be fairly easy, Bardic Music could give a +1d bonus to allies hearing it, the size of the die could start at +1d4, and go up a little at higher levels.
Inspire competence: +1d4 to skill checks
Inspire courage: +1d4 to melee attacks
Inspire ferocity: +1d4 to damage with melee attacks
Inspire defiance: +1d4 to saving throws
etc...
And Paladins could just be removed.
I'm thinking this would be a good start.
Isn't it always?