SUMMARY Currently when you'd like to change the shell, e. g. from Plasma Wayland to Big Screen, you need to log out, select the shell, enter your password and log back in. If you use different accounts for each shell, that makes sense. However if you use the same account, IMO things could be sped up: The user selects in some interface to which shell they want to switch and Plasma would do everything automatically: 1) Save the session 2) Logout (is this even necessary?) respectively shut down the shell 3) Log back in (if a logout was necessary) and start the new shell 4) Restore the session That way if you switch from your computer or laptop screen to a TV, you could quickly enter the Big Screen shell (whose session gets restored) and when disconnecting from the TV, you could quickly enter the usual Plasma Shell again (whose session gets then restored).
IIRC we had this in the past in KDE4 when changing shells on the fly was all the rage. I wonder what happened to it. Maybe it's even still there and could be brought back as a user-facing thing without too much effort.