Here are my ideas about a SMG3 game
The hero of the Mushroom Kingdoom, always ready for an adventure or save his dear Princess Peach!
Mario's brother, he's always scared by... pretty much anything but despite this he still helps his friends.
Special: Lower fall, like in Super Mario 64 DS pressing the control button Luigi will "flutter" with his legs and fall lower, useful for reach big distances.
The princess of Sarasaland, she's a tomboy and almost anything can scare her, she's here for help her friends.
Special: Speedy run, When Daisy run she can pass falling floors without fall with them, some enemies will not notice her too.
This guy loves money and he want to become better than Mario but despite this he has joined him to adventure.
Special: Super Punch, Wario shots a punch which can destroy heavy blocks (like in SM64 DS) and kill some enemies faster.
Red Luma returns from the previous games, she has the sames powers.
Like in SMG2 you can use Yoshi on quests.
Rosalina She appears in the comet observatory, she's the guide like in the first game.
Princess Peach Princess Peach appears as a Damisel in distress again.
Toad/Captain Toad The Toads (and more notably Captain Toad) appears in this game as cameos/helpers.
Lumas Lumas appear as cameos and sometimes helpers.
Note: I've not counted minor supporting characters like the ones which appears in some levels only.
Bowser The first main antagonist of the game, he has invaded tons of galaxies and kindapped Peach as well, he want to marry her and makes her his queen.
Tatanga The second main antagonist, he has helped Bowser and he want to kindap and marry Daisy.
Bowser Jr Bowser's son, he helps his dad and the mysterious alien during their plans.
Note: These are the main antagonists on the castle/final world levels, other antagonists are present in the game.
Like in SMG1, the hub of this game is Rosalina's comet observatory, but this time it's bigger (there are present the laboratory, a Yoshi's place etc.) unlike the first game for open the map you need to access to one point (like in SMG2).
In the hub there are some special levels and secert stars too.
World 1: the solar system,
similar to World 1 and 2 from SMG2. Leader of the World Bowser Jr and his minion.
World 2: the stars, a world filled with stars and comets (it's the place where the stars spettacles happens) Leader of the World Tatanga.
World 3: the milky way, Similar to world 4 from SMG2. Leader of the world Bowser.
World 4: the mysterious planets, a galaxy filled with strange planets (water only, green only, fire only, a own-life planet etc.) Leader of the world Bowser jr and his minion.
World 5: the crystal galaxy, A galaxy filled with crystals and mysterious lighs, these crystals are really hard to destroy and Bowser and Tatanga use it for block Mario. Leader of the world Tatanga.
World 6: the timeless galaxy, A galaxy where the times looks blocked: time loops and extinct things appears here. Leader of the world Bowser
World 7: the Black Hole, Similar to world 6 from SMG2. Leader of the world Bowser jr
World 8: nothingness, We're inside the black hole where everything looks dark and lost, Bowser and Tatanga main base is here. Leader of the world Bowser and Tatanga.
World 9: special world, an after story world with special levels.
World 10: champions world, the last and shorthest world, there are all nostalgia levels from old Mario games and a final level.
There's a party in the Mushroom Kingdoom, here an exclusive star raining event is happening, Peach has invited Mario to join her.
Mario comes to the Peach's castle, here, at the door of the defensive walls meets Luigi, he came here for see Princess Daisy.
Princess Daisy is in the shops, she's buying things for the party.
Wario has followed Mario and he makes jokes to him in front of everyone.
Mario meets Peach in front of her castle, Peach is always happy to see Mario and she's smiling to him while she's greeing. There's dance, there are songs, there's food, everything looks perfect... at one point a big dark cloud was covering the sky. Bowser and Tatanga revealed themselfs, they have teamed for destroy the universe! An army full of robots pilotated by Bowser's minions and Tatanga's minions was destroying everything. The main robot (pilotated by Tatanga) has kindapped Peach (following Bowser's orders), he also tryied to kindap Daisy without any success, Rosalina and her observatory comes(she was watching the event in the sky of the Mushroo Kingdoom) has tryied to stop them but there was a mysterious power to strong for her. Bowser and Tatanga goes away with Peach.
Mario has decided to follow them and save the princess, Luigi, Daisy and Wario (after a small disscussion) are gonna help too and Rosalina and Lumas guide the heroes inside her starship and starts the journey.
A mysterious force was destroying the universe and Mario and his friends needs to found the Stars and restore the power of Rosalina's Comet Observatory.
Bowser and Tatanga have created some crystals wall, they can be destroyed only with a certain amount of Stars.
Between the galaxies 1 - 4 the 3 main bosses were acting in a strange way: they were talking about "it" and that his power was enough big for destroy the universe.
In the 5th galaxy it was revelated that "it" was the "Shadow Star" and his deadly powers: the dark star can destroy anything in just seconds, Bowser and Tatanga are using this power on themselfs for become immortal and rule the universe for ever and conquer every existent galaxy.
In the 7th world Bowser's Jr fortess was destroyed and everything was going great, unfortunately the black hole has attracted Rosalina's observatory inside.
In the final galaxy Bowser and Tatanga secret world was revelated, a world full of strange things and monsters. When Mario reach Bowser and Tatanga they've obtained all the power of the Shadow Star and they became extremely powerful.
Mario and his friends will defeat the deadly duo and Peach was saved and she gives to Mario a kiss, the Shadow Star power was lost and the black hole get destroyed forever. Mario and his friends will come home where everyone was celebrating the victory of the adventure and the party has started again.
Mario and Wario rivality
Since the beginning of the game Wario has followed Mario and he made jokes on him in front of everyone, after Bowser and Tatanga kidnapped Peach and the group was alredy formed by Mario, Luigi and Daisy, Rosalina has invited Wario to join, "ugh, why I should help you?" Rosalina said that It's a good thing and it's a gesture of extreme generosity, but Wario still doesn't care, after the mention of some treasures hidden in the galaxies Wario was alredy in the starship.
Mario and Wario have shown rivality during the adventure, (of course only for Wario's side it was a rivality), they were often quarrelling too.
In world 4 Captain Toad got a weird fever, Rosalina needed some special herbs to create an antidote and since Luigi and Daisy were helping Rosalina Mario and Wario were obligated to go in Bowser jr's fortess for finding it.
in this castle they were fighting more than ever, their fight and screams have alarmed Bowser and Tatanga troops, they were following them! Mario and Wario ran away but they fell into a trap, here they were waiting to a moster sent to Bowser Jr for eat them.
In this occasion Wario still makes insults on Mario but when he said "why do you hate me so much Wario?" Wario has revelated that while Mario got a lot of love and friends while Wario was always alone and without a home,the two have made a temporainly truce, they have escaped and the monster was beaten. Mario and Wario has come back with the special herbs for the antidote, Captain Toad was healed and everyone is congratulating Mario and Wario.
Luigi and Daisy's crush
Luigi was watching where Daisy was at the Peach's party, he was watching her when she was doing shopping.
Luigi was always in love with her since he met her in a Golf Club but he never told her this because he was to shy.
Daisy had a crush on Luigi too but everythime they went out Luigi made a disaster and Daisy was thinking that sometimes he was doing it purposely "maybe he doesn't like me...".
After Tatanga has kidnapped Peach he has tryied to kidnap her too but Daisy was able to defend herself and he went away.
Tatanga has attacked Daisy several times during her adventure, he was in love with her and he wanted Daisy to be his queen, of course Daisy was not excited for the idea.
While Tatanga was planning more projects for kidnap Daisy, the princess and the green plumber have intereached so many times, Luigi was trying to tell her about his crush, but the bad luck striked again on Luigi. Daisy was not telling him because according Peach the man is the one who needs to make the first step.
On world 6 tatanga has tryied to kidnap Daisy again but he failed (again), he has realized that Luigi and Daisy were in love with each other, he was jealous and he has decided to kidnap Luigi, bring him to Bowser's case and blow it up with Luigi in. Mario, Daisy and Wario have tryied to stop him but Tatanga has hurted Mario and Wario, they needed medical assistace and they were not able to go in the base for save Luigi. At this point Daisy has decided to go alone despite anyone though it was too hard for her.
Daisy has met Bowser in the main room where Luigi was trapped and after a big slap Bowser was defeated, the base has started to explode and there were no exits! At this point Luigi toke Daisy and told her that he loves her, Daisy said the same and they had a kiss. Luckily an exit was created with the fall of a wall, Luigi and Daisy come out and they came back to the starship.