Side note: This is not going to be a real thing. Just an idea.

Extended story overview

Mario & Luigi get news that something weird is happening inside Peach's Castle's sewage system, so they travel over to fix the problem. Toadsworth explains that weird and strange noises have been coming from inside the sewer system, which is locked behind a heavy door that only Mario & Luigi have the key to. 

As the Mario Bros. travel to the door and unlock it, they go through the sewage system and locate the noise. There seems to be a strange portal that leads to another dimension. Curious, Mario & Luigi both touch the portal, when suddenly they black out. When they wake up, they notice a weird, purple colored, slimy counterpart of Mario running towards the exit. They follow this counterpart and eventually encounter him. They find that the clone was with a Luigi-like version as well. 

They then begin their first battle. The instructions for how to battle the opponent is shown on the screen with arrows, instead of the long tutorials where an npc explains how to battle.

After their first battle, they make it out only to find that there were tons of purple, slimy counterparts (that toads had named "The Slorde")  of mario & luigi running around ruining peach's castle. There were too many of them to fight off, they just had to escape as the clones destroyed the castle. Peach and a couple of others had escaped to a secret massive bunker, where they would be safe. However The Slorde had covered the top and they could not get out.

The Mario Brothers escape and head for Superstar Road, where Starlow lives. They hope to find an answer from Starlow for how to defeat The Slorde. Starlow has one answer, a map that was once torn and scattered across the mushroom kingdom and given to protectors. The map, when combined, contained the way to get to Star Tower, where they could come in contact with Starlow's ancestors, which, with their help, they could defeat The Slorde before it's too late.