Oh we can only choose 1.. Well I would like to see Mario Party go back to MP6's format or 7 since I am more familiar to 7.
I would like to see Paper Mario: TTYD's style to return.
Another Double Dash styled game and a mario Kart game with Gliders, underwater and anti-gravity but maybe something new.
Mario Kart 8's battle mode is nearly a literal mix between Mario Kart Wii and Mario Kart 64. The only thing wrong is the courses themselves. So keep that style of battle mode, have some arenas.
But my option? Mario Party being like Mario Party 6. And it needs the return of Brighton and Twila.