Mother 3.5 is a canceled expansion set for the also canceled Nintendo 64 video game, EarthBound 64. Shortly after EarthBound 64's development shifted from the Nintendo 64DD to the standard Nintendo 64 console in 1998, Mother producer Shigesato Itoi made mention of development on a 64DD expansion disk. Few details regarding Mother 3.5 were explicitly made available, but IGN estimated in August 1999 that it may contain an entirely new quest and additional characters, but utilize the same engine as EarthBound 64. IGN had confirmed they would be playing the unfinished EarthBound 64 in Japan later that month.
In 1997, an IGN report on EarthBound 64 stated, "Real-time internal clock of the 64DD to affect gameplay. Realistic, graphically intense world; Compelling storyline; Unique individual experience guaranteed for each player; Ability to play different characters; Because the 64DD is much larger than carts, EB 64 will feature more speech, music and textures than other N64 games." Though the game's development was ultimately shifted to the standard Nintendo 64, it is likely that Mother 3.5, itself being a 64DD project, would have extended to these features.