This is a concept mod: I will be making this mod along with a team!
When you begin a world, you spawn with a menu in front of you. You have many options to pick.
Terrestrial Lifestyle: A Vanilla lifestyle but with limitations on what you can do. You can't climb vines, slower in water, ect. No other info needed.
Aquatic Lifestyle: You are extremely fast in the water, you can dig, build, and live underwater. Once every time, you would need to resurface to get air. You are very very slow on land and weak.
Semi-Aquatic Lifestyle: Mix of Terrestrial and Aquatic. You are slower on land and slightly faster in water, you can be able to live on land and water. Every few minutes underwater, you would have to resurface.
Arboreal (Tree) Lifestyle: This lifestyle includes faster climbing, ability to be under trees, climbing on most blocks, and much more. Inspiration is taken from Primates that live in trees. You are faster at climbing ladders and vines. You can easily live on a tree and meet many of your daily needs.
Terrestrial-Arboreal Lifestyle: This lifestyle mostly takes inspiration from Gorillas and Chimpanzees. You can walk normally on the ground and be normal on trees, your heavier, so you wouldn't be able to do as much tree-activities.
Glider Lifestyle: A Glider is some animal that is arboreal and can "fly". This takes inspiration from Flying Squirrels. You can fly from tree to tree, climb up trees, and walk on land slightly slower. Very much like having an Elytra
Flight Lifestyle: You can infinitely fly, or somewhat. You can live on land or in the trees. You can't climb on trees but elevated places are good homes. You can launch and fly into the air and travel nearly anywhere!
Airborne Lifestyle: You never land, you only can fly and be on land is very risky and slow.
Subterranean: Dig through dirt, gravel, grass, sand, and other blocks. Often a side picks with Terrestrial, Aquatic, Semi-Aquatic, or Glider.
Herbivore: You can exclusively eat non-meat products. Tallgrass, flowers, leaves, etc.
Omnivore: You can eat nearly all food-stuffs ingame. Omnivore has few restrictions.
Carnivore: You can only eat meats and some fish.
Piscivore: You can only eat fish, you can use a button next/in water while fishing for a cool down, or find dead fish items.
Ovivore: You can only eat eggs, feathers, and bones. Very limited but very filling.
Insectivore: You can find bugs within the grass, plants, or nearly anywhere.
When you begin, you will get a menu to choose the lifestyles that you want to pick. Right after that, you choose your diet. Whatever you choose, you will be placed in the nearest place that you are well adapted for. Aquatics will be spawned in water, Semiaquatics will be spawned on beaches, ect. This will account for diets too, so Subterranean Insectivores will be spawned in deserts or savannahs. If you don't want that, you will be spawned in the seed or random seed. You can be able to change the lifestyles later in the game. Each environment will get a semi-overhaul with ores and crafting for each environment. So you don't go mining for anything as an aquatic, you can do so in different ways.
It could be interesting if you made it so you had to unlock the different lifestyles after living as one for a while (since being able to fly right after spawn is a little op, imo). For example, you have to start as a more basic, "tier one" lifestyle like the terrestrial lifestyle, then you can upgrade to "tier two" (such as terrestrial-arboreal) after a certain time of play, then tier three (e.g. flying). Maybe have this as a config option, or something? idk. But great idea, I'm looking forward to seeing how it will turn out!
Hopefully this mod does take off. It is extremely complicated and probably scares people away. That could be interesting, some sort of tier system. I was thinking maybe instead Flight Strength could increase overtime, representing aging or something.