It would play exactly like 7, only that you don't have to split a controller in half with someone else this time. All minigames would be in 8 player 4 teams ffa, 2 vs 6, 4 vs 4, 2 vs 2 (replacing duels) and whatever other categories you can think of.
If they can fix the server connection on finding other players for online Mario party it'd be nice. It's nice that once you find someone if they leave for whatever reason they become ai controlled
It has potential, I think, but online multiplayer is a must for something like this, and also the requirement of CPUs filling in for empty slots.
Mario & Sonic's 2020 game has online multiplayer and some sports support up to 8 players, but it is marred by needing an amount of players that makes it unlikely to start the game. So like a simple game of 100m? You need at least four human players, but how many players actually play this game to get that amount of people? Mario Party is a more popular series, as much as some might not want to admit, so it shouldn't have trouble filling in, but it is still needed to be able to support less human players if this unfortunate thing happened.
Though, if Mario Kart and Splatoon are of any indication, eight players is a good number to enjoy a game, so this would be a good feature to develop if they are up for it.