...well... a topic I was working on and fell asleep before posting lol
Characters first since this is the easiest.
This dream roster will feature 20 characters with all characters in past playable positions returning, plus a character who some people might find surprising but surely fitting.
Mario - it ain't a Mario Party without the namesake! Dashing in red and always the go to hero for princess saving or party hosting, Mario's got everything covered for this blockbuster!
Luigi - mean and green, the younger brother has always been there from the start as well! Well trimmed moustache and fear of the paranormal intact, Luigi has his opportunities to shine and show that the party ring is his limelight.
Peach - the archetypal princess just loves parties. Not tea parties. Mario Parties.

Graceful yet feisty, her charm makes any party so much more alive - but perhaps attracts Browser's attention as well...
Yoshi - being the most loved of playable characters to date, Yoshi is ready to spread happiness in this latest iteration of Mario Party! Perhaps Yoshi eggs could make good 'fireworks'...
Wario - a rival to the man in red, this generally greedy adventurer thou D's up the cast of super regulars, having only missed a stint in Advance. He's ready to steal the show from the others and prove that going for gold is best.
Donkey Kong - oh look who's back! The great ape had a big time out since the fifth game, but during that interval he went toe to toe with the nefarious Koopa King and came out on top many times! Such strength will be sure to keep everyone safe!
Daisy - spunk and sass are just some of this idol's specialties. You know you've got a popular party when another celebrity princess starts digging the tomboyish activities!
Waluigi - ensemble dark horse, lanky, cranky, and deceptive. It takes all kinds to make a Mario Party... and to be fair, people are starting to accept him these days. Perhaps its his time to climb out of the shadows...
Toad - the eternal retainer and diligent helper of the Mushroom Kingdom. A sense of can-do and strength to boot makes organizing any party a breeze! Princess protecting...? Maybe not so much.
Boo - spooking your party since the beginning, Boo loves causing mischief and unhappy party goers with its coin and star stealing shenanigans. Being playable has tamed it a bit, but it still concocts plans for chaos on a whim.
Koopa Kid - this poor guy has been neglected of late. His appearances in the old days were under the guise of Baby or Mini Bowser but it was in the GCN era where we remember and love his romp. And now his renaissance has arrived.
Toadette - a hiatus of 5 years has done nothing to dampen Toadette's zeal for partying and otherwise having a blast. She may not be as homicidal as everyone thinks but surely she's got secrets planned ahead...?
Birdo - a bit of a controversy in the Marioverse, but hey, who doesn't love controversy?! The crew certainly don't mind a gender-disputed egg spitting dinosaur. Neither should you. Just enjoy the party okay?

Dry Bones - there's no bones about it, the undead love partying as much as we do. Dry Bones probably needs a throat lozenge for all the squawking it'll do as it parties hard - in the meanwhile keep its funny bone active.
Hammer Bro - the elite of the Koopa Troop still find a day at a party the most relaxing thing after pay raises (if they get paid that is). Certainly more interesting than being a generic mini game obstacle. Jo-jo!
Blooper - inking everything on sight is something Blooper's learning to control. Floating in midair it is mastering. Being accepted by the fan base as a viable character might prove too big of a fish to fry though...
Koopa - keeping a stoic composure isn't easy for any minion. But Koopa is different. An unsung hero of board logistics in the earliest games, and a business boomer otherwise. He's got the dough to make the party the best it can be.
Shy Guy - some mysteries are best left unanswered. Especially the mystery of Shy Guy's mask. Of course, no-one sees value in pulling it off - no point creating a tension point between partygoers! And Shy Guy would prefer... not being so shy.
Kamek - this wily wizard decided it would be fun to join Bowser's latest conquest of the Mini-Stars. Sadly beating his boss/nurse child/protege in mini games has kindled a desire to join the party crew for real this time. Those senile wizards!
Bowser Jr (new) - following in his fathers footsteps doesn't have to strictly mean 'cause havoc on the board when called for'. He's decided that causing havo as an actual party player would do just fine. And besides he knows the ins and outs of Mario Party. He better do his dad proud!