Now, admittedly I'm a masochist so this might make it too difficult for a lot of players to be arsed with it, and coding might be a bugger.
I've played a fair bit of UnReal World and I like how that handles survival. It's a true survival game. Food isn't everywhere, and for the first however many games you play you're likely to starve to death, if you don't wind up dying to a bear.
To this end, I'd like to see some changes (or options to add these for a more realistic survival experience).
1) Most animals are faster than players, or players no longer have infinite sprint time. This'll make active hunting more difficult and rely on strategy more.
2) Animals have line-of-sight, possibly adding sound/smell mechanics, so as to allow you to sneak up on animals before attacking them. Sound mechanics could be that if you step on a certain block type, it'll produce sounds that'll alert animals within X radius. For example, if you step onto a stick while sneaking there'll be a snapping noise that alerts animals within 10 blocks of you. I'd say while walking too but that'd get annoying when you're running through a forest.
3) Animals have endurance, and being injured slows them. This allows for persistence hunting, or if you get a shot off from a distance with a bow you can more readily catch up with the injured animal.
4) Tracks/trails. Some system which lets you see tracks that animals that have recently passed through have left. If an animal has been shot, occasional temporary blood splatters are left on the ground that allow you to follow the animal.
5) Passive hunting methods. Traps, ranging from small to large. A pit trap is simple, dig a pit and hope an animal falls into it. Allow the player to cover the top in sticks/leaves to cover it and make animals consider it normal terrain, but it collapses under them when stepped on by a large animal. Use rope for large snares to catch larger animals, and twine for small snares to catch things like rabbits.
1) Active fishing. Active fishing is using a rod or spear to "hunt" for the fish. If fish mobs are added, making them ignore you so long as you're still so you can stand by the edge of a waterbody, wait for the fish to come to you, then spear them. If you're in the water, they swim away faster than you can catch them. The fishing rod acts as you would expect. You cast it, the fish comes to the rod, you pull on it.
2) Passive fishing. Fish traps, really. If rivers with flow are added, allow the use of funnels with a trap at the end to direct fish into traps.
1) Make it more long-term, high yield, high but sporadic effort. You have to make a large field, you can fertilise it to increase growth rates if you want (much like it is now), and harvesting takes some time, but so long as it's large enough the field will give you a lot of food and resources (such as hay for cereal crops, which can be used to feed animals or for construction). There's effort needed to plant a large field and keep it fertilised if you want, and to harvest it, but there's little effort after that excluding chasing off vermin.
2) Changes to crop growth based on temperature/humidity. If you're somewhere arid, only certain crops that can survive arid climes can grow. Less suitable crops will take longer to grow, and some will simply fail. Same for temperatures and such. Humidity can be overcome by using nearby water bodies to keep the soil moist, but temperature issues are there to stay. This could also play into seasons, with temperatures changing and so only allowing certain crops to grow optimally a certain times of year. If it gets too far outside the temperature range, crops will die so you can't use them as an improvised food store by keeping the crops going throughout winter.
2a) Add more crops suitable for certain climates that give certain resources. One thing I'd like to see is optional crops like coffee, herbs, spices, tea etc. that you can add to certain meals (or perhaps are required for more complex ones) or make drinks from so that it can be advantageous to settle in some places, and hopefully stimulate a bit of an agricultural trade network in multiplayer.
2b) Add agriculture traders so that you can access seeds (and animals) that aren't from the local area.
3) Animal husbandry. Already in game in a basic format, and from what I can gather it'll probably be changed anyway. As it is now, only certain animals can be "tamed" and it still takes some generations to tame them. Allow them to be used for more than just meat, fat, bones and leather, such as allowing sheep to be sheared for wool, cows/goats for milk, etc. And they *need* feeding. If you're somewhere grassy it's fine, so long as they have enough room they can just feed off the grass, but if there's not enough room or no grass (for example, because it's winter) they need feeding to keep alive.
3a) Some animals require shelter from cold/heat. A great big shaggy highland cow like creature is fine in the cold, but chickens aren't and need access to a coop. A highland cow would suffer in the heat and require something for shade, like a tree or artificial structure.
More preservation forms would be nice, too. We've got brining, pickling and curing, but adding smoking and drying would be good. Drying can only be done in dry climates for obvious reasons, so either places that are so cold there's no moisture in the air (Or over winter if it gets dry enough then) or in a desert. Smoking takes time but only requires a smoker and wood.