VB Mario Land (aka: Mario Adventure)

Manufacturer: Nintendo
Genre: action/platform
Were to have been released in both Japan and (then) America.

A 3D Super Mario Brothers 1 remake starring Mario & Luigi fighting Wario!
Revealed to the press at the Winter Consumer Electronic Show (CES, which is the present E3) in 1995.

Game specifics, quotes from Richard Leadbetter of NMS at Las Vegas CES '95:
"Other games showed that Virtual Boy at least has some decent titles in the pipeline. The star of the show was undoubtedly the new Mario game. This wasn't the Mario Bros arcade game as revealed at the Shoshinkai show. This was Mario action as we like it. Bearing an uncanny resemblance to the Super NES Mario World (but with superior sprites, believe it or not), this game looked superb. The Virtual Boy hardware is put to great use in this game, with Mario being able to walk into the background and explore whole new parts of each level. There's even an overhead section a la Zelda!"
 +
"Mario! This untitled Mario product looked like being the strongest Virtual Boy title at the CES - Some amazing sprite-scaling and Mode 7 type effects are in evidence should Mario fall through a hole. However, most of the 3D effects are used to show Mario wandering in and out of the screen in a new addition to the Mario formula."

Miscellaneous Ingame:

Sequenced Ingame:
(all the following pictures are part of the first level demo and are in chronological sequence)

*It looks like Wario is upto his old tricks again, go Mario!

*And into the pipe we go! Where will it lead, a secret room perhaps?

*Nice Zelda-esque overhead view in this room

*Don't forget to collect the gem next to the candles

*Out of the room and into the background

*As always with Mario platform adventures there's plenty of jump action

*I'm getting a Super Mario Bros. 1 jump-the-flagpole flashback here, awesome. Isn't that LUIGI?!

*Tudedudedudu tudedududu tudelelululu tulelule da da DA DA! ... Hey! Where's my fireworks?

*Level 1 completed, what lies ahead we may never know :-(

*A really early version of the game had the classic Mario Bros. arcade mini-game (like in Super Mario Bros. 3)

Important note: VB Mario Land did not change into VB Wario Land, why you ask?
Because VB Wario Land (at the time still Wario Cruise) and VB Mario Land were BOTH seperately shown to the press as demos in the month the VB was released in the USA (July 1995);
Here is what was said in the press about the "untitled Mario game": "It looks very promising and uses the 3D to good effect. Strangely, it was absent from the E3."
To make the situation even more confusing (but is meant to clear things up): the now named Mario Clash was originally meant to be a "platformer" a la Mario Land with "bonus games", the "platformer" is the now unreleased VB Mario Land, and the "bonus games" in that game were expanded to the Mario Bros. Arcade game we now know as the released Mario Clash.

Is the existence (of a prototype) confirmed: Yes...
... A Demo has been seen for show on TV screens at the Winter CES 1995.

This was confirmed by multiple English and American magazine journalists (one of which Cyril Lachel) who all saw (with special 3D goggles) & wrote the demo was on show in America, most notable is an article from the English Electronic Gaming Monthly magazine and I quote:

"Yes, every new Nintendo system has to have a Mario game as a sendoff. Virtual Boy gets a 3-D remake of the original title involving Mario and his faithful brother Luigi."