What I am going to do here is make a post for each class, and list the options from Tasha's given for that class, denoting which are allowed as substitutions (most) and which may be trained, per Xanathar's, considering it as a tool proficiency.

Note that many new Tasha abilities, such as the expanded spell list or other ways to use charges of an ability, I am considering as substitutions.

Training:
Xanathar's page 134
Basically it takes 10-Int Mod workweeks. You have to pay for living expenses plus 25 gp a week for the Instructor (assuming the Instructor doesn't want more). Also, you have to find an Instructor, which, for these purposes, means a member of the class with the ability. Druids, for instance, may be hard to find and keep in one place for that long.

There may well be events happening during this training that will affect it. See Xanathar's

Training may not need be done all at once, but may be broken up in many cases.

Expanded Spell Lists:
All are good and may be taken without restriction (unless something about the subclass restricts it)
This message was last edited by the GM at 21:03, Sun 04 Dec 2022.

Barbarians:

Not a long list
Training:
-Primal Knowledge. This is basically any proficiency, a bit more than normally allowed by Xanathar's, but as a one shot class ability, it'sfine. Teacher needs to be a Barbarian, however.

-Instinctive Pounce

Bards:

Training:
-Magical Inspiration.
-Bardic Versatility: Cantrips only. Skills are sufficiently covered by Jack of All Trades. Each time this is used is another training project.
This message was last edited by the GM at 01:40, Fri 29 Apr 2022.

Cleric:
Subsitutions:
-Harness Divine Power
-Blessed Strikes

Training:
-Cantrip Versatility (Each new Cantrip)

Druid:
Substitution:
-Wild Companion

Training:
-Cantrip Versatility (Each new Cantrip)

Fighter:
Substitutions:
-Fighting Style Options
-Maneuver Options


Training:
-Martial Versatility (Per replacement)

Monk:
Substitution:
-Ki-Fueled Attack.
-Quickened Healing
-Focused Aim

Training:
-Dedicated Weapon

Note that Monks are limited to the East and probably not available in most games.
This message was last edited by the GM at 01:49, Fri 29 Apr 2022.

Paladin:
Substitutions:
-Fighting Style Options
-Harness Divine Power

Training:
-Martial Versatility (Per replacement)

Ranger:
Substitutions:
-Fighting Style Options
-Deft Explorer
-Favored Foe
-Primal Awareness
-Nature's Veil

Training:
-Martial Versatility (Per replacement)
-Spell Casting Focus (Teacher must be Ranger (with training) or any Druid)

Rogue:
Training:
Steady Aim

Substitutions:
-Metamagic Options
-Magical Guidance

Training:
-Sorcerous Versatility (Per replacement)

Warlock:
Substitution:
-Pact of Talisman (Access may be limited by some specific Patrons)
-Eldritch Invocation options

Training:
-Eldritch Versatility

Wizard
Training:
-Cantrip Formulas