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The second one first - I think it is a good design principle for D&D. If I want to explore the differences in various approaches to magic, I'd probably use a different system that is better suited for it.Does 5e's magic feel same-y to you? The spellcasters?
And do you like the design principle (spell equivalent) of 5e?
Heh...I actually read your OP just before I had to jump in the car, so I was thinking about it while I was running some errands. The idea to have all spells be exclusive is one I've had before, I just had to think for a while if (Arcane, divine), (arcane,divine,druid), or (arcane,divine,druid,bard) made the most sense as dividing lines. Obviously, any decision comes down to a matter of taste.So good, Twosix! It's almost like ...you've thought of this before.
A lot of this is good stuff TwoSix!Let me think...how to approach your concerns while leaving the bulk of the game intact.
1) Dump all the spells in the game into a spreadsheet. Label each spell as either a wizard, cleric, or druid spell. No overlap. Those are the new spell lists.
2) Eliminate every damage cantrip in the game as a base choice. Cantrips become fun non-combat effects. (Cantrips earned from any higher level subclass features or other effects could still be allowed on a case-by-case basis.)
3) Wizards need a subtle boost to offset cantrips, make Arcane Recovery work every short rest to compensate.
4) Cleric domains are altered to only include cleric spells.
5) Make bards a half-caster class using the wizard spell list. Paladins use cleric, and rangers use druid. Bards need a boost here, some more inspiration points would be appropriate.
6) Sorcerers pick their spells from all spell lists. This gives them a mechanical niche different from other casters to compensate for their low spells known.
6) Warlocks only know their domain spells. Mystic Incarnum spells can come from any spell list, just have to fit the patron theme. Gain 1 invocation/level to compensate.
7) If you want deeper cuts, I'd cut spellcasting out of paladin, ranger, warlock, and bard. You'd need to build homebrew replacements, but they exist. Alternatively, just cut those 4 classes out of the game, and turn some of the more interesting abilities into new subclasses for the remaining 8 classes.