On the cover of the Televi-Kun magazine June-July 2021 issue there is a new shinkalion unit revealed. Its name is "Shinkalion Z 800 Sonic" (シンカリオンZ 800ソニック). There is no other info about it yet as the said magazine issue hasn't been officially released.
The "800 Sonic" is likely to be the Z combination of "Shinkalion Z 800 Tsubame" and a new Zailiner that is likely to be based off either the 883 series or 885 series trains that runs the Sonic limited express service on JR Kyushu. Whichever it might be is yet to be confirmed.