I also believe Waluigi should be in the WarioWare series.
Like, even if they're not brothers, they're best friends aren't they?
The are close, but they have different goal.
Which sometimes lead to conflict.
Waluigi is jealous of Luigi.
He considered himself as Luigi's rival.
He always want to surpass him in anything.
He doesn't care how much money to spend in order to defeat Luigi.
That last one is the one that bothered Wario most.
Wario on other hand, doesn't care of rivalry at all.
All he care is treasure and making more money.
He's stingy, like uncle Scrooge, but not that rich.
He's not considering himself as Mario's rival.
He isn't interested in Mario if there is no coin or money involved.
That's why Wario never ask Waluigi to join Warioware projects.
First Waluigi isn't interested since it's not Luigi-related.
Second Wario doesn't trust Waluigi as his business partner since the way they spent on their money is different.
Their relationship works best in sports events.
For Waluigi, Wario could have all the rewards he wanted as long as Mario-Luigi duo is defeated.
Different goal that doesn't conflict each others.
