Let's Get Real - the DM's responsibility is balance

[Cheesegear]

What shouldn't spells be able to do at all? Ie what are the limits of magic?

I think spells need to be neutered.
I think magic items that do spells need to be more plentiful and accessible.

Wizard: At Level 5, I'll pick up Lightning Bolt and Fly, also all my Cantrips are better.
Barbarian: At Level 5, I'll grab a Javelin of Lightning and some Winged Boots, also I'll get Extra Attack.
DM: ...Umm, no Barbarian. You can't do that.
Barbarian: What do you mean? Extra Attack is on my class table.
DM: No I meant you don't just get a Javelin of Lightning and Winged Boots.
Barbarian: But the Wizard just gets Lightning Bolt and Fly?
DM: ...Yes?

As it stands, there isn't any room for "you need to do that with mundane means". Everything that can be done mundanely can be done as easily, usually more powerfully, and certainly more reliably by a spell.

That's what I mean.
I believe that the back part of the PHB needs to get cut in half. At minimum.
I also believe that magic items that do what gets removed from the PHB, get added.
I also believe that 'mundanes' should be able to cast from Scrolls, so long as the character has spent at least one long rest reading the scroll. Kind of like an attunement. I also believe that Scrolls, or spell instructions, should be able to be used to imbue magic items, temporarily and/or permanently.

Magic needs a significant rework, as a game rule. No. It doesn't make sense that a character with Int 8 should be able to cast spells. But, if a Fighter finds a Wand of Fireballs, they should be able to use it. The spellcaster who already casts Fireball, does not need a Wand that casts more Fireballs.

And if there isn't an exact spell yet--just wait. Each new book brings dozens of new spells. Spellcasters all get new options, often without even leveling up, whenever a new book is published. Non-casters (and non-casting options)? No. Usually only at most a feat. Or something where they'd have to take a new subclass.

Are the creatures in Volo's and Mord's significantly more powerful than the creatures found in the MM? Not really. CR is CR.

Characters just get new options, often more powerful than what they already have. 'Cause that's the point of power creep and that's how you make sales.
But DMs get nothing on their side of the table. A Goblin is still a Goblin, there's no rule to make more powerful Goblins. Technically yes, you can make Goblins more powerful, but that technically increases their CR. What I'm talking about is simply improved monsters without increasing their CR. Just like how each book increases PC power, without actually increasing their level.