To be honest, a new
Mario Party could very well be revealed as early as next year. We need to give Nd Cube some time to develop their ideas for the next game.
Of course, this does bring to the table of whether or not Nd Cube wants to continue the
Mario Party series since they aren't exactly Hudson who kept pumping out
Mario Party titles almost every year since 1998 regardless of sales.

Still
MP9 sold well, and I'm sure that there are plenty of fans waiting for a 3DS or even a Wii U title as well (myself included); thus, Nd Cube may very well develop one soon.
I'm fairly sure that the main reason there was a decrease in MP titles recently was probably due to Hudson having lost the rights to the series and closing down (so a new developer needed to be found).