Considering that systemd is working on Varlink, I would be interested to see what KDE Plasma would look like with Varlink instead of D-Bus. I have no doubt that D-Bus is going to stay for a while, but I feel like there’s a point where it will have to go, even if it’s in any year past 2100.
There’s also a blogpost from a while ago regarding QtVarlink. I should look and see if there’s been any progress on it. I might even see if I have anything to add. :^)