Cleric domains/spheres. So there will be some spells that sit in a universal domain/sphere that all Clerics get, but the othere domains/spheres that Clerics do/don't get access to are determined by the god that they worship.
I want a Cleric of Pelor and a Cleric of Kord to have more differences than the holy symbol they carry and the domain spells they can cast. They should feel and play differently.
+1. Two priests that worship two very different deities should have very different spell lists, not 95% of the same spells plus a few domain spells.
Real vancian spell casting and scaling by level spells.
I really want this too... I almost can't believe that nearly nobody cares, that the current spellcasting rules aren't as vancian as they used to be. This to me is one of the defining mechanics of D&D, meaning that it's a huge part of the shared experience of playing D&D (for about 34 years) rather than another rules system, but sadly WotC did not include it in the list of defining mechanical elements, so as a consequence, keeping it in 5e was not a priority. I thought this would even prevent many older-editions grognard to recognize 5e as D&D, but seeing that almost nobody brings this up, then apparently I must be wrong.!
I fully get that some people are still attached to it but the as far as I'm concerned , "real vancian casting" always felt clunky and counterintuitive, even (or especially) when I started playing with AD&D. It certainly wasn't anything like what I thought magic should feel like. If anything, 5e is still too vancian for my taste.
I never liked Vancian casting either. Not only was it clunky and counterintuitive, as you said, it was the primary cause of the imbalance between spellcasters and non-spellcasters. In order for 5e to be a more balanced game, it simply had to go. I think 5e's approach is a very well designed system and is a good compromise that preserves the overall style and good things about Vancian casting while fixing its worst problems. You still have spellbooks. You still have the strategic element of preparing spells (and I could argue it's more strategic than it used to be, since you can prepare fewer spells at a time than before). But at the same time, you don't have the rigid inflexibility that forced you not only to guess what spells you might need in a given day, but how many times (ugh). Because wizards don't have to specify how many fireballs they prepare each day, this allows them to give wizards fewer daily spell slots overall, which greatly helps with the caster vs. non-caster imbalance of the past, especially at higher levels. And then there's at-will cantrips and rituals, which I really love, and allow wizards to continue to contribute even when their daily spells have run out (or when they're trying to conserve them).
The way I see it, what has always made a DnD wizard a DnD wizard was never the exact details of his resource management. It was that he carried a spellbook, used strange gestures and arcane words, and most importantly - the spells he used. Fireball, magic missile, invisibility, fly, and especially the named spells, like Mordenkainen's Sword. Those are what, to me, have always distinguished DnD wizards, not the exact technicalities of how they managed their daily spells. And in that respect, I think 5e wizards are still very much the same, iconic DnD wizards there have always been, just with better mechanics and IMO more balanced and more fun to play.