Since the most recent game wasn't any number or reference to the console it's on, there's a small chance that it won't be 10 or U, although I reckon it'll be 10, unless it's on 3DS.
I personally think it could be Mario Party 10 if it does still follow the same formula like every of those in consoles. Even though Mario Party 3DS had a name, it still doesn't mean that Mario Party 10 will have the same thing. It's what I think, but we never know if there will be one and if it will really continue going this way too.