I say that the next Mario Party installment should have a different graphics style. Maybe cel shaded cartoony graphics? I still stick by my idea of having multiple Nintendo characters. The Mario Party to end them all!
I guess after about a few months there could be DLC for more boards/minigames. Have a vastly different artstyle, but keep both the playing styles of Mario Party 1-8 and 9. With the Wii U disc capacity, it could be the biggest ever! With gorgeous new maps and maybe even a contest to help design new minigames. To be honest, some of the games may be stale, but I'm sure that with some help, it could be the best Mario Party ever!
Could even use the game pad's mic for some of the gameplay seen in 6 and 7. A new graphical change would improve the look of the Mario Party games quite a bit, seeing as how I felt that 4-9 had bland and rehashed character models. The models seen in those seem somewhat lifeless compared to the look of the N64 games. It's neat how much personality they could fit into a small amount of polygons and low res textures!