There won't be one.
The issue is branding. The Mii brand is analogous to the Wii (it's Wii upsidedown). But with the failure of the Wii U and, to a lesser extent the 3DS, Nintendo is eliminating anything with these systems. Look at everything regarding the Switch and you'll see this
- No Virtual Console
- No Miiverse (Nintendo shut it down despite the 3DS still being sold)
- No Mii game at launch (Wii launched with Wii Sports, Wii U with Nintendo Land. 1-2-Switch ditched the branding)
- No Warioware game (3 of the 4 systems had a Wario Ware game that showed off the system)
Nintendo is redoing everything with the Switch. This means Miis. They are only used on the Menu and for Mario Kart 8 Deluxe (because the Wii U game used them). I think Flip Wars was originally using Miis as well. So in almost every regard, Miis are being killed off.
"But what about Tomodacchi Life," it did well, but not amazing well. Miitopia kind of flopped. Miitomo, Nintendo's communication app, was a dud too. Every other mobile app did better and all of them featured Nintendo's characters. Nintendo is not oblivious to this fact.
The issue is branding. Nintendo doesn't want to promote Miis as much as they want to promote their IPs. Look at ARMS. It did worse than Tomodacchi Life, but Nintendo can sell toys and a graphic novel. They don't have that with Miis. The truth is, no one cared about Miis the character. They cared only about the game. This worked for Wii Sports, but it flopped the minute the Miis became anything more than an avatar.
The truth is that Miis are being killed off just as anything related to the Wii is. The Wii U flopped, so those brands are dead (also noticed how Wii U was killed immediately while 3DS lumbers along). With the Wii brand dead, the Miis die too. They will be phased out by the next system. No one really cares about them, not even in Smash (which they are considered bottom tier characters and no one even bothers to argue for their legality). They're dead.