@Riot, Would it be possible for Riot to do some crossovers with other IPs?

I just got to thinking about it. Contrary to what people may think, the most popular MOBA in China is not LOL or DOTA. It's called 王者荣耀. I think the english title is Glory of Kings. Most of the characters are historical figures from the various dynasties, including the 3 kingdoms era. But they also have characters from SNK's Samurai Shodown series (Samurai Spirits in the East). They have Tachibana Ukyo and Nakoruru from the SS series, and if I'm not mistaken they also have Mai Shiranui from KOF. I do know they have Ukyo and Nakoruru.

Would it be possible to implement characters from other IPs into LOL? Or would that be a legal nightmare?