I will start with "DarkSide" the canceled Santa Monica IP that GoW III director was working on for ~4-5 years upto its cancelation in 2014 right before an E3 reveal that was rumored.

it was basically a game that was probably mostly a shooter with a supposed melee combat system.RPG and vehicles.  the earth basically stopped rotating somehow and one side of the earth is dark. am not sure about other details but it just has really cool art. 

so probably a mix of Mass Effect and no mans sky in a post apocalyptic earth

they spent ~100 million dollars on it . again rumored. but not surprising due to the scale it seems it had  and the almost 5 year dev time. and they had battlestar galactica writer on the story of the game.

http://vgleaks.com/leak-darkside-game-ps4-sony-santa-monica-decided-not-develop/

StarCraft: Ghost. It looked really fun, mixing stealth gameplay with the world of StarCraft.

Command & Conquer: Generals 2

How about the N64 version of Mother 3?

Not so much an entire game - but many of the 2nd screen features of Breath of the Wild on WiiU I guess were bagged at some point. After enjoying having the 2nd screen map in Wind Waker/TP HD, ACIV, Splinter Cell, XCX, and Arkham games, I really was really looking forward to how BotW would build upon that.

Final fantasy 7 remake

*jumps back in time machine heads back to the future*

imo if you ask me. its a vapor ware demo

e.g lily bergamo, eight days, agent, deep down

Sony does it more than microsoft tho. 

do I have proof. no. but everyone is quiet about it. usually when a game gets canceled you get a reason  why. for example the Darkside game was canceled because supposedly it was too similar to no mans sky and horizon zero dawn. and they had a bit of trouble with the combat system. 

either that or the game was not gonna work on the xbox console. the base one at least. because the demo looked a lot better than the xb1x version of MHW. and that game had multiple beasts at once. a technical demo that was probably running on PC that they knew it won't run on a console . I mean it was lagging on the PC in the demo 

Scalebound, assuming it remained Xbox exclusive. Combined with Sunset Overdrive, it would put me one step closer to having enough reason to buy an Xbox.

Metroid Dread

The original RE4 before it was changed into something akin to Gears of War. Yes, I know Gears came afterwards, but I am making a point. I'd like to see what the original game would have been like before it was scratched and Capcom pulled a SMB2 on all of us.

Mega Man Legends 3.