Is Nabbit the newest mainline Mario character?
>Mario & Luigi
>Mario & Sonic: Rio
So? They're newer. One of them has appeared for a second game.
If the question were about who is the newest SIGNIFICANT Mario character then maybe you could say Nabbit, though whether or not he's significant is highly questionable.
Well since the roster was made before Super Mario 3D World launched, i'm guessing because of that and Captain Toad that we'll see him in Smash 5.
So yeah, just gotta wait 6 or something years.