Offtopic: Phosh on PostmarketOS (usually Plasma) actually seems to be a thing. (added video at very bottom of Chronology).
Not sure if they just want to have choice or like the Wayland-only ways of Phoc.
OTOH, I’m not sure if Plasma mobile will be available in PureOS right away.
Same on Desktop for me, but Gnome 3 always felt like mobile to me and I like what Purism is doing, so they might convert me for mobile 
It also seems likely that Phosh will initially work much better.
On-topic:
Not many things will be customizable in the first months. I expect most-requested things to be added, and if somebody makes a nice patch/pull-requst, Purism will likely say yes.