From what I remember, Konami explained it in a reddit post.
“R is a throw back feel for the Devs. to when games could simply have a letter and allow that letter to carry many meanings. It symbolized Returns & Reborn because this game is such a homage to SNES and SB3. But it also mean Rangers, to represent the diverse and colorful group of Bomberman characters who patrol the galaxy.”
So in short, Returns, Reborn or Rangers. Take your pick.