That's a horrible idea.
There are still so many characters they can add in DM and GX. And they've been intentionally withholding content as of late, which doesn't help.
DM: Duke Devlin, Tristan Taylor, Marik Ishtar, Solomon Moto, Espa Roba, Rebecca
GX: Zane Truesdale, Chumley, Sartorius, Yubel, Lyman Banner, so many others - the GX lineup is kind of petty by comparison
Give us GX characters and story events. Actually use that part of the world.
Then you can add your third franchise. Although I'm not sure that's a great idea anyway, considering if they have more than two worlds, a lot of places are just going to feel like dead features.
5ds pros:
(1) A new mechanism that will refresh the game
(2) New archetype with various playstyle
(3) Support for Blue-Eyes,Karakuri,Six Sam and Fabled
(4) More gems