They need to make an iPhone game, seeing as Wario has always been at home as a portable game series. Not some runner game like Super Mario Run, which I think for Nintendo was a step backwards. They need to make a real platformer with real control of direction, and preferably, no buttons plastered on the screen. A unique set of controls based on gestures would go a long way. The iPhone would certainly provide the platform that would reach out to a lot of people. But another problem is making Wario as a character likeable to casual gamers. The first step is to get rid of the "gross wario". I just read the translations to the jp warioware manual, and the focus on toilet humor I doubt anyone could find funny. Certainly he could never be taken seriously as a character if his story revolved around fart jokes. Wario is best left as a brute who does what he wants and is determined to get his money back or find the treasure. Wario as an explorer is an appealing character design, I really liked it when Wario had the explorer helmet in Virtual Boy Wario Land. Wario is also defined by his character flaws, and the gameplay reflects that. I like to think of Wario as making a lot of mistakes, but managing to be cool even when making a mistake: "Wario got infected by a zombie! But hold on, it just opened a new area! Cool!". I think this lack of death where death would normally occur is one of the biggest strengths of the series, and needs to be taken to its full potential in a new game. It encourages discovery and forgives mistakes. The transformation system makes enemies a helpful or harmful thing depending on the situation. Wario Land Shake It did not take these mechanics very far, simply borrowing some old transformations from the previous games (although if you count the run booster as a transformation, that might count). I don't even think Wario Land 4 introduced new transformations come to think of it, but they definately did a good job with the puzzles. Maybe the solution is not to add new transformations but increase what can be done with the ones that exist.
I would say that transformations, shoulder tackle, and throwing in isolation lack combinational depth. The mechanics themselves cannot be combined in any unexpected way to create new situations. For instance, shoulder tackling is always the same, it never gets any deeper. I'm either breaking a block, or defeating an enemy. Throwing is a little deeper, but tends to be used toward simple spacial puzzles. Merging mechanics has the potential to create very interesting gameplay. That's what I think Nintndo needs to do with the series: take everything great about the previous games, expand upon these mechanics under a simple control scheme, introduce the game to a new audience, and reshape Wario to be more appealing to that new audience.