Mario & Wario: Nearly released in Americas and Europe
Mario & Wario would have been the second appearance of Wario, however it was never released in North America and Europe, it had come out in Japan in 1993. This obscure game is actually the second appearance for Wario after he took over Mario's castle in Super Mario Land 2: 6 Golden Coins. Mario & Wario was developed by Game Freak, which is the main developer of the Pokemon series.
Mario & Wario - Japanese Box Art
Wario has resumed his role as antagonist against Mario and the crew. However in this game, he has hilariously dropped random objects, such as buckets, on the head of Mario (alternatively you can also play as Princess Peach or Yoshi). Since Mario can't see with something on his head, he will walk forward without knowing what obstacles or hazards are in his way.
You as the player take the role of a fairy called Wanda, to help "guide" Mario to get to safety to reach his brother Luigi at the end of the course so that Luigi can remove this thing on his head. It's a simple, yet very creative concept. It's a platforming puzzle where Mario is only controlled passively as he will blindly walk along whatever path that is along the way. Mario & Wario is one of few games that were ever designed to use the Super NES mouse.
In the age of touchscreen devices, a reimagined Mario & Wario would make an excellent puzzle game on mobile devices such as smartphones or tablets. Mario vs. Donkey Kong: March of the Minis is quite similar to Mario & Wario, though in this case there are multiple characters to control instead of just one. Elements of this game have often been replicated by some Super Mario Maker stages where you have to guide an enemy across an area.
One strange thing to note is that the Japanese release of Mario & Wario was entirely in English, no localization would have been necessary which means a western release would have been very straightforward.
What happened to Mario & Wario?
Based on numerous magazine and newspaper reports, Mario & Wario was unveiled at the CES trade show in Summer 1993.
A report from Bill Provick in the Ottawa Citizen on July 17, 1993 stated that Nintendo of Canada was going to release Mario & Wario in September 1993 for a price of $69 CAD. The same article also mentions Super Mario All-Stars which was released that July.
The game never came out in Canada that September, nor anywhere else in North America for that matter. This report makes it sound as though Mario & Wario was to be imminently released, but held back at the last minute.
According to SNES Central, it was previewed in Nintendo Power Magazine in September 1993. There was even a Kellogg's cereal contest in Canada that had a giveaway of a few SNES games including Mario & Wario, the cereal also included a picture of a never-seen-before box art, in addition with an implication that the game was delayed to some point in 1994. If the release was not certain, it would be rather strange to have a new game marketed in this manner.
Unfortunately, there hasn't been any official word from Nintendo as to why Mario & Wario was cancelled and was never released on the Super NES in North America.
References in other publications
From archive.org, there were actually several publications (besides Nintendo Power) that featured Mario & Wario, which means that it was a game that Nintendo had seriously considered releasing it to western audiences.
Mario & Wario was actually featured on the cover of VideoGame magazine in Brazil in November 1993 with a featured review - showing that the game probably should've been released that year. It would be a bit odd to make a magazine cover and feature on a game that was merely speculated to be on the way. This magazine also mentions the "extra" mode which are the bonus stages that are unlocked later in the game.
VideoGame Magazine (Brazil) - November 1993: "Help Mario defeat Wario, the craziest villain"
It was also mentioned twice in Super Play, a UK-based Super NES magazine in September and October 1993.
Super Play - September 1993
Super Play - October 1993
Video Games Magazine in Germany also had a brief mention of Mario & Wario in their September 1993 issue stating that the game was scheduled for release in December.
Video Games Magazine (Germany) - September 1993
Of course, at the end Mario & Wario for the SNES was never released in Europe.
It's quite a curiosity why Mario & Wario never released outside Japan despite many indications it was on the way - the signs seem to indicate a last minute cancellation. Could it be that Nintendo didn't want another Mario game to steal the spotlight from Super Mario All-Stars which was released around the same time? Or is it that they had already given up on the Super NES mouse?