Bellow I will list what is required for 100% completion in RDR2:
Main story
Finish all 107 main missions.
Side Activities
Complete 10 stranger missions.
Complete 25 chance encounters.
Encounter a single Gang Ambush and survive through it.
Complete 5 bounties.
Interact with 5 special characters.
Complete 6 gang hideouts.
Visit the theater.
Play each game at least once.
Exploration
Discover and inspect 50 different animals.
Acquire 10 different pieces of equipment.
Discover and collect 10 different species of fish.
Discover 10 horse breeds.
Discover and use 48 different weapons.
Encounter 6 gangs.
Obtain 20 different wild plants.
Discover 5 shacks.
Character
Finish all challenges.
Reach maximum level of Health, Stamina and Deadeye attributes.
Reach maximum level of bond with horse.
Collectibles
Discover 1 Point of Interest or landmark.
Catch all Legendary Species of Fish.
Pay your respect to the 9 Graves.
Find 20 Dreamcatchers.
Discover and inspect all 30 Dinosaur Bones.
Collect 1 set of Cigarette Cards.
Observe all 10 Rock Carvings.
Complete 1 Treasure Chest hunt.
Complete all Hunting Requests.
Acquire all Flora and Fauna.
Miscellaneous
Craft recipes for all six different materials.
Execute four different types of robberies. These include home, train, coach, and shop robberies.
Take down 5 Legendary Animals.
Perform a bath.