We've had this discussion a while ago but, with AoCF being released on the Switch, I could see the possession mechanic being implemented into Smash, in which case we would get 2 characters in one slot. Kinda like Pokemon trainer and the 3 different Pokemon.
Given, it doesn't have to be Marisa (I favor her as the potential partner), as some people brought up Yukari as Reimu's main partner, due to her presence in AoCF's story, the chance for a duo is there.
On the topic of Echo: I'm clearly in the "please don't" camp. Marisa and Reimu are so different that I can't see them work as echos without making huge sacrifices to their unique abilities. They would have to be made way to generic because their trademark moves just don't work for each other. It would be a waste, in my opinion, to dumb down one characters uniqueness just to get another one in. Especially if a multi-character is possible.