The menu contents as revealed during the Nintendo Live 2018 event. Options for "Adventure Mode", "Spirit Board", and "Collection" can be seen. The Adventure Mode tagline at the bottom reads "Save fighters and spirits in a world that has lost everything."
Although this mode appears directly on the main menu, when Ultimate's main menu was initially revealed during the August 8, 2018 Nintendo Direct, the name and symbol of the mode was obscured.
This menu item was revealed the day after Halloween (October 31), itself celebrated as All Saints' Day in some parts of the world (as well as November 1st), and before All Souls' Day (November 2nd). From October 31st to November 2nd is also Día de Muertos (The Day of the Dead) in Mexico. Halloween, All Souls' Day, and Día de Muertos are all associated with "spirits".
As a possible reference to this, the November 1, 2018 Direct revealing this mode states that the Spirits referred to are "not the spooky kind", as Halloween is a holiday commonly associated with "spooky" ghosts and spirits.
Spirits are similar to Mirages from Tokyo Mirage Sessions #FE. Both are based on characters and they both help other Characters.