Expanding on this again, I'd specifically want a tactical turn-based RPG like this where Bowser uses his own army (of Goombas, Koopa Troopas, Hammer Bros., and the like) to take out other armies and kingdoms led by other Mario villains (as mentioned in the above quote). Battles would take place on an expanded chess board (like Pokémon Conquest), and each minion would have different attacks with different ranges and sometimes different effects. Fighting and/or defeating other minions would give experience and eventually levels to that minion, making them stronger and more fit to battle. Each type of minion shares the same level, and there's a limit to how many of each you can bring into battle.
For example, Goombas would attack the space in front of them and more are allowed into any one battle, while Koopa Troopas would attack the three spaces in front of them with a piercing strike (but hitting walls and such would halt the attack and leave them vulnerable for a turn) and less of them would be allowed into any one battle (in comparison to Goombas).
Red Koopa Troopas would attack the three spaces before them, but divert the attack in any possible direction if a wall is approached. If there is no other way, they'd stop on the last vacant space they passed during the attack. Hammer Bros. would attack from 2-3 spaces away, kind of like archers and Artillery units in Fire Emblem and Advance Wars. Fire Bros. would hit every other space ahead of them from the spot they throw from with their fire balls.
Paratroopas would be support units that can also perform weak attacks if need be. They can carry units across rivers or up hills, and while not carrying other units, can only be hit with certain attacks (eg. thrown hammers from a Hammer Bros.). They can only attack if they're above an enemy unit, and dive-bomb them from up there. After impact, the Paratroopa will bounce in the direction of the user's choice, after which the Paratroopa is vulnerable as they are stuck on ground level till their next turn. Bowser would probably be able to recruit some of the enemy units during the main story, allowing for even more unit variety.
Besides just the combat, the terrain of the maps you invade could have exploration features simultaneous to the action. Switches to be hit with various means to allow new paths to be crossed, boulders that can be hit to hurt anything in its destructive path, traps that can either hinder or assist you during battle and probably more. And there should definitely be online if this all can be worked out.
Plus this would probably be on the Wii U if this ever happened, so the GamePad can easily be used for it.