While the roster may have been supposedly decided in 2015, things change over development. Someone from within Bamco has come forward to me with info on characters being put into the game, some I think most of you are going to find odd. The Smash Ballot wasn't completely for nothing though, as it determined which characters ultimately got into the game.
The characters are: Metal Mario, Banjo-Kazooie, Mega Charizard X/Y (Echo), K.Rool, Dark Samus (echo), Decidueye, and Issac.
First things first, Simon IS in the game, but he's an assist trophy. He was a playable character in the planning stages, but the team dropped him down to an AT.
Banjo wasn't even a thought until 2016, which is when he was added due to sheer requests for him, much like Ridley. The Wonderwing isn't his final smash, and his up b is nothing anyone would expect.
K.Rool was added thanks to overall global requests and thanks to the Smash Ballot.
Decidueye was picked over Sceptile due to it being the most recent grass Pokemon starter. His Final Smash is Sinister Arrow Raid from Sun and Moon. Speaking of, Mega Charizard is in the game due to Charizard mains in Smash 4. It is the only PKMN Trainer Pokemon to get this honor, and yes, you can play as Y version and it too has 4 colors.
Issac was added to the game early on thanks in part to fan requests. It should be noted Issac's green hands are nowhere in his moveset.
Dark Samus was added as a more offensive version of Samus. Her F air has only one hit box for example and it does kill at low percents. Her Final Smash however, turns her into Metroid Prime.
Metal Mario is actually a throw back to Super Smash Bros 64, complete with his stage remade in Smash Ultimate. His side B is a diving tackle from Mario 64 and his down B is Mario Tornado. Metal Mario also has his step sound effect from 64 and falls like Fox did in Melee, but won't die at 260+ percents like he did in 64. Ask me anything else you would like to know, especially DLC.
