A Mario RPG that's unique from the groundwork that SMRPG, M&L and Paper Mario have done?
I think I'd make it a platformer, but a slower one. With many RPG elements and real time action. Buttons for actions like jumping and power ups for example. Many of the typical Mario Power Ups being collectable items indeed but with permanent function if you choose so. Like grabbing a Fire Flower in Mario.
Lots would rely on platforming, positioning, but all in real time. Mario would increase in power over time, like increased jump capacities and run speed and amount of durability and name it, but nothing extreme. I would prioritise skill over stats, a skillful player would be able to reach Bowser and rescue Peach without leveling up potentially. Just like in Zelda Breath of the Wild and Tears of the Kingdom basically.
So the power scaling would need to be adjusted to this principle. And the environment and collectable power up items would need to be of influence too to the battles. As well as regular jumps and stuff from enemies like Koopa Shells being easy to exploit against enemies. Kinda like in the regular Mario platformers huh. I'd also scale the items compared to the environment. Maybe like having levels on the power ups themselves? Level 1 Fire Flower minimal to level 5 Fire Flower maximal for example. I don't know.
So yeah in a way I'd try to make this game appealing to speed runners, completionists, lovers of grinding, and add an engaging story if you actually take time to go and liberate the castles the Princess isn't in. And I'd try to keep it as Mario as possible, but with a slowed down engine to support a more strategic approach, yet not too much as to dumb down the platforming aspects.
My approach would be to combine the best aspects of the Mario series while also making it a traditional complestionist RPG with a deeper story, which is optional to go after.