1) Realistic Mode: Would be a realism slider or an advanced button with checkboxes. "Difficulty" would be changed to "Realism" with the lowest setting being "Creative" the highest setting being "Starve". May include temperature effects like freezing to death. Maybe even feature a weight limit? Like you can only carry a certain amount of items or you walk slow (Notch never mentioned any details).
2) Persistent Block Damage: Damage done to blocks (The "Block Crack"), lasts 10 seconds before healing back.
3) Seasons: Notch said there are technical issues. Leaves change color. More rain in Spring/Fall. Colder in winter causing upper layer of water to freeze and snowfall.
4) Liquid Slime Tile: Runs like water or lava. Enemies are made of that material and spawn there (Likely Slimes).
5) Fire Creepers: Have a smaller blast radius but spawn in lava or whatever causes fire.
6) A Mob that doesnt move when viewed. Kind of like Boo's in Mario. They only follow you if you are not looking at them.
7) Ghasts were supossed to spawn near Nether Portals rarely in the overworld!
8) The Nether was supossed to compress your location from 16:1 instead of 8:1 (Could just show 2 nether portals 16 blocks appart and one 8 blocks appart or something like that, to show it off).
9) Crafting Areas with Prerequisites: Like a cooking tabke or a stove or something. It requires a chimney or some other blocks nearby. The more you have that "fits" the block, the better it acts or something like that. IE; Crafting Table that only lets you craft certain items if a Blast Furnace is nearby, etc. Side Note: If you have played terraria you will know what its like!
10) Torches Burn off after a while since it is too easy to make dungeons safe. To balance it, mobs will be more sensitive to light. Then he will add an Oil Lantern that burns forever. That's like a high tier torch.
11) Considered having a opt-in randomly upload what I build mode for lots of content. Could use smoke/mirrors and structure blocks to simulate this.
12) Netherrack: "it's hard to draw red moss in few pixels". Netherrack was supossed to be red-moss on stone.
13) Beer because according to Notch "it looks really cool". Not sure if it will have the same effects of beet. Use breweries to make beer which will probably be like the furnace. You start it and keep it running and afrer a while you get a mug of beer or something.
14) Notch is unsure of "customizable armor". Says it sounds good but then you would not know the kind of armor you are wearing, so he said he might have it so you can add an insignias on armor.
15) Someone suggested TNT arrows, and Notch liked it. Mentioned there is a lot of potential. You combine an arrow and TNT to make an arrow that explodes, and an arrow with a bucjer of water so you can create water springs!
16) Seige weapons are "definitely going to happen". Mentioned Cannons because they are "really tricky to use and would be kind of fun to see." Load them with different ammunitions so you can have burning boulders or you could launch most things like creepers. You can load it with any entity or block and let it launch it (You don't have to literally do every mob/block. Maybe just creepers).
17) More enviromental hazards. When asked about Quicksand he said "Might belong, actually". Also said when you go swimming there would be black prickly things that could damage you (Sea Urchins, probably).
18) Square Rainbows is something Notch considered adding.
19) Only Obsidian was supposed to be a floating block. "Perhaps all floating chunks of blocks should fall down". Floating islands would need obsidian veins to stay afloat.
20) Secret Doors that look like rocks!
21) Entities you could "attach" blocks to, and they could move. So boats and elevators/things that move which you could design yourself. Maybe you could use falling block entities in the "shape" of a boat and walk over a barrier path, letting it follow you. Just for simulation purposes. Or /clone elevator that moves, but you can also change the design. Smoke/mirrors, etc.
22) Engravings: "Paint your own signs and chisel your own engravings". Maybe you could add a chisel item and make a stone made of armor stands. Right click to kill off some of the armor stands! Or when you right click it teleports you to a blank chisel room in creative mode that lets you break appart the blocks. Of course, we would need to kind of force it to become a certain structure and have Tyler pretend to break away the stone. Maybe its constantly being cloned so Tyler can't break the blocks, and the rest of the blocks are filled in with stone falling blocks. Tyler right clicks with the chisel, it kills the falling blocks, but not the stone. Only really need one example.
23) Large Monsters or "Mega Beasts": Something notch wanted to add. They will be a challenge no matter how equipped you are, but are never annoying/uncontrollable. Maybe just make a Giant boss battle or a 3-D model in its hand slot to make it look like a giant monster. Or use Optifine to make the Giant look different.
24) Planting a Flag (Costs gold in survival) claims an area of land. Monsters can't spawn or die from age in that area. Only selected players can build in that area.
25) Future Mobs would be fantasy themed: Kobolds/Goblins. He liked the idea of finding a goblin village. They would be small and swarming while Trolls would be hude and can throw destructive boulders. Trolls could be retextured/maybe slightly remodelled with optifine, Iron Golems that attack/throw Zombies holding a model that makes them look like a boulder. If one of the other ideas in the list is "not possible", you could split this into 2 numbers!
26) Originally there was going to be No stackable food since it is "too powerful".
27) Probably something like Wet Sand: Stays floating but then it eventually falls.
28) He wants to add Coins because he likes them. Give them a use, maybe replace emeralds with coins or something like that?
29) The ability to cook multiple foods at once is being considered. Maybe retexture the Dropper UI and give it a new texture that looks like a furnace, so you could have multiple cooking slots or something like that.
30) Prefixes or Mob Modifiers such as Flaming Birds, Shadow Creepers, Giants, etc.
31) Water Gates: Like trapdoors but lets water through.
32) Hedgehogs are being considered... May hurt if you step on them.
33) Certain nights the moon will be red or something like that, which will indicate a zombie horde. For this one you could just retexture the moon red and have the time set to midnight when you press the button. Daylight Cycle would be off.
34) Throwable Torches are being considered.
35) Sharks as a passive mob.
36) Clouds slow the player down, being physically active with the player. For this you could have a showcase area at a high Y level and give the player slowness... Or a custom cloud model (Just a slightly transparent white cube on an armor stand) so you can execute commands from the armor stand.
37) Notch's favorite mob suggestion is the Pigmen.
38) Probably going to have additional inventory slots, like a cloak on your back. For this you could use an invisible Minecart with Hopper (That is renamed) that is constantly teleported to you when you are wearing the "cloak". So when you right click, it opens up extra inventory slots.
39) Enchanted Armors may possible be a prefix and a suffix like "Burning Armor" which auto ignites or something. Though Notch said "That is not a good one", so maybe there are some other examples you could come up with.
40) Notch may add a 4x crafting table.

If some of the points above don't work, here are other things Notch Suggested...
Alt 1: Summon creeper armies, etc. "Rituals/summoning". Maybe add a Totem of Creepers that summons baby creepers or something. I personally would avoid rituals but that's just me.
Alt 2:  Notch said "Dragon's are definitely planned. This one is a bit loose since he said Dragon's not Dragon. You could stretch it and add a Red Dragon though he said this before the Ender Dragon was released.

Alternatively you can check the link where all these points come from: https://www.minecraftforum.net/forums/minecraft-java-edition/discussion/116353-minecraft-facts-big-list-of-what-notch-has

To find a substitute. Though please let us know :)