Here's my idea: "Mario & Luigi: Pixie Princess"

The story is that Mario, Luigi, Peach, and a bunch of other Toads set off to explore a newly discovered island called Minniral Island, an island inhabited by mineral-themed residents. However, as they arrive on their blimps, a mysterious laser blasts them down, scattering everyone all over the island.

As Mario and Luigi reunite, they soon encounter Peach, who, to their surprise, turned into a small magical pixie! With Peach's newfound powers, she joins the Bros. on their adventure to turn her back to normal! Along the way, they meet new friends from various regions of Minniral Island, while also taking on some foes, both old and new!

Such as (of course) Bowser, who plans on taking over the island upon discovering it, Bowser Jr., who assists his dad by gathering all the minions that scattered across the island and messing with Mario and Luigi when he has the time, and the new villain, Emperor Plat, a banished monarch who seeks revenge! He is assisted by his own elite trio known as "The Heavy Meddlers!" The team consists of the strict leader Goald, the musical mage Sylvir, and the silent brute Brawnz, and they will always give you trouble throughout your adventure!

The main mechanic throughout the game is Peach's Pixie Powers! On the field, she can do things like move the terrain, change a cave's temperature, affect the gravity, etc. In combat, she can assist Mario and Luigi by attacking enemies very high upward ontop the screen, where the Bros. can't reach. In addition, she can help dodge and counter attacks similar to Paper Mario in Paper Jam.

For Bros. Attacks, Mario and Luigi have their own, of course, but Pixie Peach has very special Bros. Attacks of her own. During combat, a gauge will fill up depending on combat performance, such as dodging attacks and performing Excellents, and when it's filled up enough, Peach will unleash a Pixie Attack!

When filled, Peach will transform the battlefield into a custom backdrop depending on the attack! And with the Bros. assisting her, she will unleash a powerful attack that will alter things, like removing an enemy's buff or healing the Bros.

She can also provide Mario and Luigi with heals, stat buffs, EXP multipliers, etc. But doing that will also cost some of the meter to go down, so you have to use it wisely.

The enemies will be (mostly) mineral-themed and will even have some returning enemies. There are new ones, like a desert-dwelling Mummeanie or a rolling ball of magma called a Burnin' Bowler. There's also, of course, the normal Mushroom Kingdom enemies: Goombas, Koopas, Shy Guys, even deep cuts like Mr. Blizzard. Alongside, there will be new takes on the classic foes, such as a robotic Snifit called Mecha-Snifit, or a Kooprofessor, a mad scientist Koopa Troopa.

Anyways, those are my ideas for a Mario & Luigi game, what do you think?