Homebrew Capstones

Some of the class capstones are rather disappointing, let's fix that.
Artificer: +1 to saves per attuned item. End infusion as a reaction to stay at 1 hit point.
Barbarian: Your STR and CON increase by 4, maximizing up 24.
Bard: At the start of your turn, you regain one use of your Bardic Inspiration die.
Cleric: Your Divine Intervention ability is now guaranteed to succeed.
Druid: Unlimited Wild Shape.
Fighter: Four Attacks in one action.
Monk: Monk Damage Die is d12. Once per turn, you can spend 3 ki points to turn a missed attack roll into a hit.
Paladin: Oath Feature - Avatar Forms.
Ranger: Once per turn, when you make a weapon attack roll, you can choose to automatically hit.
Rogue: Once per long rest, you can treat an ability check as a 20 on the die.
Sorcerer: Select one spell from your Spells Known that has a duration of 1 minute or more and targets a single creature. Once per long rest, when you cast this spell, you can choose to make the effects last until dispelled or until you use an action to end the spell. You do not have to concentrate on this spell to maintain it.
Warlock: As an action, you regain your Warlock spell slots. You can use this feature once per long rest.
Wizard: You can concentrate on two spells at once. Whilst you are concentrating on a spell, you can concentrate on another as long as one of the spells you are concentrating on is 3rd level or lower. Failing a concentration saving throw, causes both spells to drop.