I haven't played any of the mario party games since 5. As for 9 I watched some trailers for it and it looks interesting, but I don't imagine that I'll be getting the game. Still waiting for them to add an online mode to the series, hopefully they add it in the next mario party 10.
I've heard many people make arguments against the addition of the online element as to why it hasn't been added, about how it's meant to be played in person, and that it wouldn't be good since people would disconnect if they're losing and so on. Though I'd say that the main purpose of it is to be able to play with people that you know easily and at any time, and for people who you only know online. Who in which case wouldn't disconnect during the game, and that you may not have a chance to play the game with otherwise.
Haven't come across anything much related to the story mode, but the new car game mode looks like it could be good. Compared to everyone getting all split up throughout the map, keeping all the action in one place like that could make it more lively.