I think these are too far (more in the 4h one way range)
I've added the others.

Wittenberg (Luther), Wörlitz and Dessau (Bauhaus) are easy to reach from the Harz.

On the other hand Quedlinburg and Goslar are easily reachable from Saxony Anhalt (let's say Dessau as a centre), so I would add them there.

Dubrovnik made it. :D You should add the Venetian Fortifications.

What about Kopenhagen?

Peloponnese Hotspot
I notice that this hasn't been assigned a "centre".
Tripoli is the geographic "centre" of the Peloponnese and all of the current connected sites are arrayed around it and is therefore the most obvious
However there are other WHS which are within a day return of either Tripoli or another location. I had started by looking to see which WHS had been connected to an "Athens" Hot spot - only of course to discover that there isn't one!!
We visited Bassae from Athens and returned easily in the same day by rent-a-car. Someone using Tripoli as a centre could easily get into Athens and back by car - and could pick up the Monastery of Daphni as well (as we did on our trip out to Bassae). They could even drive up to Delphi (via Hosias Lukas) and back - but a better "centre" would then be Corinth.
So there are potentially 3 extra WHS for a hotspot connection somewhere around Corinth/Tripoli - Parthenon, Monasteries of Daphni etc and , at a push - Delphi

I've added the center + the 3 extra WHS