EDIT: @Berordn, even though I don't want Mii's in the game, I would love to play as Tom Nook.
His Final Smash would be enslaving the other characters, so they pay off their debts.
For Ice Climber, I believe their inclusion was due to Sakurai wanting to place in a character that would represent the one-off early NES/Famicom games (Ice Climber, Balloon Fight, Excite Bike, Clu Clu Land, Urban Champion, Devil World, etc.)
I believe reading that Balloon Fighter was considered as well (and Sukapon for Smash64), but Sakurai wanted a twin character, hence the Ice Climbers were chosen, since they aren't from a full solo game.
Regarding G&W and ROB, both of these characters represent Nintendo's early history, and have likely been included to show the whole spectrum of said history, and to honor its roots.
I know Smash characters don't always have to be from current and/or popular games, but they have to add some big names. So my question is of all the Nintendo characters left that aren't in Smash yet who would be the most popular, well known, and crowd pleasing?
To put it very simply: Villains and support characters. Characters like Ridley, K. Rool, Dixie Kong, Toad, Bowser Jr., etc., that aren't actual main characters but still popular household names. Also, Miis and series that have not been included yet such as Animal Crossing, Stafy, and the likes.
Of course, at this point, Nintendo is kinda running out of popular heavyweight names, so the big selling points might be third-party characters. Mega Man seems very likely (Inafune did say he'd like Rock in Smash, and Capcom is really crossover-loving atm), and I would also think that Castlevania might have a good shot at getting in.