I’d play it.

Its a real shame, the fanfic on fimfiction looks to be long abandon. Guess the guy no longer shows interest to continue.
 
I really loved that story.

We’re gonna need more buttons…

@Jarkes  
Most of what you said is exactly what I had in mind for how the gameplay would work.
 
I actually had the Mario Bros and the Mane Six as the only playable characters and the royal couple and the royal sisters as supporting characters. Every character would travel together like in Super Mario RPG and Paper Mario but you can choose whom to have out in front. For the most part, Mario and Luigi would be in the lead with a pony assisting them in combat. But the two groups can split up for puzzle-solving purposes. I don’t know how the princesses and Shining Armor’s gameplay would work both in battle and the overworld, but having them be active characters in the story is always a plus.
 
I think Spike would also be good for appearing in cutscenes with the characters for when they’re discussing ideas.

@Nintendokid88
 
Here’s an idea: Mario & Luigi, the mane 6, Shining Armor & Cadance, and Celestia & Luna would each be playable in groups of two, with their own unique Bros. Moves (and yes, they would still be called that) and field abilities, and you could swap between the different groups at any time in both battles and for puzzle-solving. However, you wouldn’t be able to mix and match the different groups.
 
The obvious groupings are Mario & Luigi, Shining Armor & Cadance, and Celestia & Luna, but I’m not sure how the mane six would be grouped. And Spike would ride on Twilight’s back and be the one who managed the inventory.

I’d buy it

Best crossover since Dan vs. FIM

@Red-Supernova  
This would be a epic game in the making. I’m down with that idea.

Someone needs to make a fanmade RPG off of this concept!

Fanfic logo, I presume?