From Academy to the Stars [ EDIT ]

Player's Handbook (2014 version)
Xanathar's Guide to Everything
Tasha's Cauldron of Everything
Sword Coast Adventures' Guide
Volo's Guide to Monsters
Mordenkainen's Tomb of Foes
Mordenkainen's Monsters of the Multiverse
Fizban's Treasury of Dragons
Spelljammer

The following are changes I require when playing certain classes.

Barbarian. Path of the Totem Warrior (Totem Spirit: Bear) only resists acid, cold, fire, lighting, and thunder (elemental damage). This is still a lot, but at least means you won't also resist force and necrotic and radiant.

Bard. College of Eloquence (Unsettling Words) uses a d4 instead of your bardic inspiration die, so the penalty isn't so huge as you level up, but to compensate it is Charisma Mod per short/long rest instead of costing a bardic inspiration die.

Cleric. Life Domain (Disciple of Life) triggers once per casting of a spell as to not be abused by spells like goodberry and aura of vitality. Peace Domain (Emboldening Bond) is a bonus action but lasts only 1 minute per use (so you might run out in a dungeon crawl situation). Finally, Twilight Domain (Twilight Sanctuary) gives temp hp equal to twice your cleric level, instead of 1d6 + Cleric Level, but only to allies within 30 feet when you use your action to activate the channel divinity. The emanation still ends the charmed or frightened condition and creates an area of dim light.

Fighter. Action surge cannot be used to activate a magic item or cast a spell. If a player is playing an eldritch knight and are not going to multiclass with a full spellcasting class, I may lift this restriction off of that player. This is mostly a multiclass change than a change to the fighter.

Paladin. Divine Smite is once per turn. It doesn't cost an anything to activate, such as a bonus action, so you can cast a smite spell as a bonus action if you really want the extra damage on a single turn. This is mostly to prevent paladin nova on boss enemies, where a paladin that is dual wielding or using polearm master can smile 3 times if they have the spell slots after level 5.

Warlock. Hexblade Patron (Hex Warrior) no longer lets you use charisma for attacks with a weapon. However, this is now part of Pact of the Blade, so at level 3 if you choose that (on any patron) you can still do this. This is mostly a multiclass nerf.

Some spells are somewhat problematic, so the following alterations are mandatory.
Quick note: silvery barbs is not here because it isn't in an allowed sourcebook to begin with so not on the approved list.

Animate Objects.10 tiny objects can get a little high in damage (avg of 65) for a bonus action, even if the objects can't bypass resistance to non-magical attacks. I'll let each player choose the nerf: (Option 1) commanding the objects is an action, not bonus action (Option 2) tiny objects option is gone, so you can animate a tiny object but it counts as a small objects for both stats and how many objects you can animate.

Armor of Agathys. the maximum temporary hit points and damage the spell does is 25 using a spell slot of 5th level or higher. This is to prevent full spellcasters from using the spell in a way the warlock never could.

Banishment. The target is not incapacitated, so it won't end their concentration, and the creature might do some self buffs or such while banished or take the dodge action for when they suddenly return.

Bigby's Hand. Forceful and Grasping hands use strength saving throws instead of initiating a grapple. This is mostly so legendary resistance still applies. Since you can do this repeatedly, it is still a good way to make a monster use up its legendary resistances.

Conjure Animals. To prevent swarms of wolves or such, you generally have to choose 1 CR 2 or 2 CR 1 beasts. You get to choose the animals, though. Brownie points if you roleplay summoning the same beasts and develop a bond with them.

Conjure Elementals. To prevent swarms of magmin, you generally have to choose 1 CR 2 or 2 CR 1 elementals. Note, however, I have stats for a CR 2 version of the 4 core elementals (fire, water, earth, and air), which I will make available. Brownie points if you roleplay summoning the same beasts and develop a bond with them.

Conjure Woodland Beings. Pixies summoned by this spell can't cast polymorph.

Counterspell. This spell is either a horrible spell or not an issue at all based on groups. I will say this: if players refrain from using it, I will refrain from having enemies use it too. If players do want to use it, one small change: After you cast the spell, you can't cast any spell of 1st level or higher until the end of your next turn as you magically exert yourself. Additionally, if the target's spell is successfully countered, they can cast a cantrip in place of the original spell, which also means they didn't cast a spell of 1st level or higher so they can cast a bonus action spell if their countered spell was their action (so they can misty step away if they wanted).

Forcecage. The cage is an object and has AC 18 and 200 HP, so creatures might be able to eventually break it.

Suggestion (and Mass Suggestion). This is generally an out of combat spell. If you are actively fighting someone, they see everything you say as unreasonable.

Tasha's Mind Whip. This targets a Wisdom Save, instead of Intelligence. As written it can create an easy default boss kill situation where the boss fails, everyone backs up, and if the boss can't do a good ranged attack or must move then they move and end their turn and rinse and repeat. Unlike command, it works on all enemies, making it perhaps too universal for the cost of a 2nd level spell slot. Not going to ban but tempted.

Telepathy. The target creature makes a Strength Save, not a Strength check. This is mostly so it doesn't bypass legendary resistance. Since you can do this repeatedly, it is still a good way to make a monster use up its legendary resistances.

Wall of force. The wall is an object and has AC 18 and 100 HP, so creatures might be able to eventually break it.

Web. Not really a nerf (in that I am not changing something), but a heads up that I interpret it the web will fall to the ground if not anchored immediately, so it can't get enemies in the air. This is so you aren't surprised if I say this later.

I allow players to request buffs or changes, with the full expectation that I can say no or limit the buff or change to help me feel comfortable.

Possible buffs or changes with a decent chance of approval are:

Changes made in the 5.5/2024 edition of D&D that help fix something or reimagines something a better way

Changing subclass spell lists to swap spells for other spells that fit the theme better

Creating new spells that are mostly alterations within a type (like a cold version of fireball or a lighting version of come of cold, but not a radiant version of fireball unless you are playing something like a light domain cleric)

Each character gets a free feat at level 1, but only from the list below, and with any adjustment provided. If you choose variant human, you can still choose any feat like normal. if you choose any of these feats after level 1, these restrictions below don't apply. These "nerfs" are only if they are taken at level 1.

Quick note: if you take a background that provides a feat, like one of the spelljammer backgrounds, you can take that feat or you can take one of the feats below, not both.

Actor (no +1 Cha)
Alert (proficiency in Initiative instead of +5)
Athlete (no +1 Str or Dex)
Charger
Dungeon Delver
Elemental Adept
Healer
Linguist (no +1 Int)
Savage Attacker
Skilled
Tough
Weapon Master