I think Wario's only presence is his likeness on the pattern of different Mario racers we see in the backdrop of certain parts of the Mario Kart preshow. I really hope they find a way to implement him someday as well.
Also, I still feel like an easy way to represent other properties would be just adding more cameos here and there, like 8-bit Power-up band spots that have an 8-Bit Link for example. 3D World did have one after all. They could also try and find a way to implement a few cameos of other IPs that would fit in the Mario universe on the Toadstool Cafe screens; I know I mentioned Kirby for that before but I think Pikmin could also work, you could probably see a few Pikmin carrying a coin or other antics for example.