New Mechanics and Features

DIY Crafting:

• You can now craft your own tools and items through the DIY Crafting App on the Nook Phone
• Crafting materials are gathered through various tools and means such as picking up fruit, flowers, and weeds, hitting trees and rocks with the axe, and shaking sticks out of trees
• DIY recipes can be unlocked through Nook Miles on the Nook Phone, gifting items to villagers, and unlocked when acquiring certain items such as the Manila Clam (unlocks fishing bait)
• Certain materials will only be available in certain seasons to encourage players in the same town to gather up resources to craft the things they want before they are no longer available

Achievements:

• Also accessible through the Nook Phone, the player can complete achievements to earn Nook Miles
• These were made to make tasks like picking weeds and catching less valuable creatures more rewarding
• Nook Miles earned can be spent on various things like recipes, special items, and services.

Hunger/Stamina:

• A fullness meter is shown with an Apple icon next to it
• Eating an Apple increases this meter from 6 to 7

• Stamina is consumed by doing strenuous tasks like picking up a tree with the shovel (decreases the meter from 7 to 6)

Fishing:

• You can now craft bait. So far the only bait we've seen so far can be crafted from Manila clams, which are dug up via the shovel on the beach. The DIY recipe for this bait appears to unlock upon digging up one for the first time
• Fishing was not fully shown in any of the videos we've seen so far (a fish taps the bobber but we haven't seen it bite down and immediately get reeled in). This could indicate there is an entirely new way that catching a fish works once it actually bites down beyond just pressing A at the right time.
• Bait is shown getting tossed in the water and a fish appearing

Breeding Flowers:

• Instead of picking the whole flower off the ground, the player may now only pick the flower part (stem and leaves still stay on the ground). This could indicate Pocket Camp's cross pollination mechanic is coming over (pick a flower, then use it on another flower to get random seeds).
• When picking flowers, they appear to take a few days to grow the flower part back.

Auto-save:

• The player no longer needs to save manually, as it is done automatically.
• You will no longer be able to turn off the system to reset your progress since your last save
• Since saves are fluid and automatic, presumably visiting a friend's town no longer will require all players to stop what they're doing so they can save, which will make starting up into multiplayer a lot less cumbersome than previous titles.
• Resetti is out of a job at his reset surveillance center, and now will fill a new role that is yet unknown

Single Save per Console

• If another user has already made a town on your Switch system, you must join that town. It has been directly stated that it will only allow 1 town per system
• Since the Switch supports up to 8 users, up to 8 characters can move into that town
• It may be possible to get around this if different regions are considered different games on the eShop, potentially allowing the player to buy other region versions to make additional towns on the same system (i.e. own NA and JP versions of the game to have 2 separate towns, 1 NA and 1 JP). This works different for various Switch games, but different Splatoon 2 region versions have separate save data, so this may indicate it will be possible with New Horizons as they are made by the same studio.

Unlockable Buildings:

• Buildings will not unlock in the same order as they do in New Leaf
• You may choose where the buildings go in your town

Inviting Villagers:

• There will be some method of inviting villagers to your town
• You will not be able to select specific villagers to invite (so you won't get a list of people to call over). It is unclear Amiibo Cards will get around this limitation or not

(Unconfirmed):

Collection Viewing:

• Previously, the player was able to view their collected bugs, fish, and deep sea creatures via their inventory menu. This was absent in the menu in the demo shown at E3, however, one of the starting DIY Recipes is "Mini Library" which may indicate that this and similar items can be used to view your collection.

Cooking:

• There is a frying pan craftable at the beginning of the game, and a cooking pot over a fire, saucepan over an outdoor gas cooker, and a large drink dispenser are shown in the E3 trailer 
• Various food items shown or referenced
  • Coconut juice is craftable, which may be only a costmetic item
  • Upon digging up a Manila Clam, it says "I could make some amazing clam chowder with this!"

Bathing/cleaning:

• An oil-drum bathtub, clothing line, and wooden wash tub are among the very few starting DIY recipes. It is unclear whether these serve some function or are merely cosmetic (perhaps the player and their clothes get dirty over time?)