I have had some ideas written down, but since I doubt I would be on the Mario movie project, might as well spill.
The film is titled "Mario". Not Super Mario, because this is the story about how Mario became Super Mario.
The film begins with an 8-bit battle between Mario & Donkey Kong. Mario reaches the top & the film transitions to 3D. Mario stops Donkey Kong, who is really Cranky Kong, from throwing barrels. Donkey Kong comes along to pick up his "pops" who is "off his rocker".
Mario & Luigi are plumbers. They live in their business, "Jumpman Bros. Plumbing", based in the capitol of the Mushroom Kingdom. Mario is a man of few words. He's humble & modest as he is a lowly plumber. Luigi is very similar, but more cautious & neurotic.
The brothers get a call from the Mushroom Kingdom to rid the sewers of an infestation. Piranha plants are popping out of pipes all over town. After a struggle, the brothers fight back the plants. They are called upon the Kingdom to report to the castle.
At the castle, the brothers are thanked for their response to the call. They appear in front of Princess Toadstool & Toadsworth. Toadsworth thanks them. Peach wants to give her gratitude, but is taboo for a princess to interact with someone as lowly as a plumber.
Princess Peach is the ruler of the Mushroom Kingdom in her father, King Toadstool's absence. King Toadstool is half Toad while her late mother is fully human, making Peach three quarters human & one quarter Toad. The king has taken most of the army with him to help with some foreign affairs. Peach does rule, but she rules according to precedent. Toads are mild, meek people that do not handle aggression. Peach rules according to their disposition; she doesn't want to upset the order or tradition of the kingdom.