A Debian Base I think is more something for the Future and not for the next Version. Maybe the Zorin crew sit already on it, who know's.
KDE Plasma ... I think this would be a too big Change. Something like Budgie what is more near to the Gnome Look and the GTK Base could theoretically be an Alternative. But then all the Work on the Gnome Customization's would be obsolete.
New Extensions and Improvements of existing One's seems for me to be the most realistic Thing. I have already suggests a Function for customizing the Login Screen. When this would come as an Extension and not build in in Zorin Appearance, I could live with that too.