In the last year or two, there have been great video channels dedicated to uncovering canceled games and their stories. Some of them even show off bits that look fairly close to polished.
This might be one of the more impressive ones here. This come from PtoPOnline, and it shows a rather bizarre title. Following in the footsteps of the LEGO games, Mega Bloks tried to make games with the properties they are signed up with. This is a one based on the Halo franchise. This is incredibly polished and at least for this slice, I assume this is very close to how this game would have ended up looking. It seems pretty similar to a LEGO game, but a little more action involved.
This is a really neat thing to see.
Take a look: