Various people including me have proposed mixing filaments by printing the filament. However, there are some good ideas here that had not occurred to me. I suspect forms of this have been invented multiple times. I know of two: My design was deliberately gradient in nature: https://www.thingiverse.com/thing:3609741 and was inspired by someone else's design for an even mix of two materials: https://www.thingiverse.com/thing:3565827