I'll bite...
SM4SH -
Finally made into a playable character with his own DLC pack. Comes with his own stage that could probably be a shifting/rotating stage that rotates around similar to Brinstar Depths. Oh yes, and he gets his own Amiibo for SM4SH too.
Mario Kart 8 - Captain Toad is playable (as a heavy character due to this backpack) and his DLC pack includes recolours for Toad. Comes with a vehicle that is based off the minecart from his game.
Hyrule Warriors - Nintendo really wants to promote Treasure Tracker and makes him a playable character. Link's in Mario Kart - forget Mario, just add Captain Toad here!
A sage? A miner based moveset with dual pickaxes? Launching his fellow brigade members from cannons? Regardless, the captain will definitely be awesome regardless.
Pokemon Omega/Alpha Sapphire - Captain Toad is the new champion. Surely has a Breloom, Amoongus, Parasect on his party. Grass types? Fighting types? Poison types? Holy shrooms, Captain Toad must be a competitive battler!
Mario Maker - Toad is playable for the jump factor. Captain Toad gets his own mode which is similar to his games, but the player can make retro levels with the Captain Toad gameplay.
Mario Party 10 - Captain Toad: "Time for a PARTY!"
Yoshi's Woolly Word - Captain Toad levels make a cameo appearance in a woolly form. Woolly Captain Toad waves from the background. Probably mining through woolly caves or some woolly mess.
StarFox U- Captain Toad randomly flies through the sky in his starshroom. Probably has some airship repair shop too somewhere.
Zelda U - Captain Toad can be called upon using the "Toad Tuner". When used in specific locations, the player can control Captain Toad and collect rupees.
Kirby and the Rainbow Curse - Captain Toad's probably just floating around somewhere... Probably makes a cameo in the background of one of the mini-games.
Xenoblade X - Captain Toad befriends the Nopons and Riki.
Splatoon - Some painting of Captain Toad is displayed in some area.
Animal Crossing U - Captain Toad moves in as a turnip collector! Sets up a Toad House too.
H-how's that? o.o Damn..I'd laugh if Nintendo does make random cameos for Captain Toad in almost all the upcoming games.