Someone suggested these two:
- Forbidden City, Temple of Heaven, Summer Palace (all in Beijing)
- 3 world heritage sites in New Delhi within walking distance

According to my Lonely Planet Beijing the Summer Palace is within the required 15km from the other 2 sites - but would you walk there? I think you have to cross some heavy traffic Ring Roads.
Like WHS Hotspot or Located in a Capital City, the goal of these connections is to provide some practical link. So: Anyone with experience walking between the 3 named sites in either Beijing or Delhi?

And another one:
Moscow: - Kremlin and Red Square - Church of the Ascension, Kolomenskoye - Novodevichy Convent

Has someone been there recently? Would this be walkable?

Dear Els,

Walk to Novodevichy from Red square will take you more than an hour and Kolomenskoye is very far from Kremlin almost 30 minutes by underground.

Thanks, Durian.
I will not add this Connection until someone has a story to proof that he/she walked there.

Brussels can now boast three within walking distance. Palais Stoclet – Major Town houses of Victor Horta and la Grande Place.

I reckon on it being about a 5.5km walk. It would be a pretty enjoyable one. From Stoclet down Tervuerenlaan, through the triumphal arches at Parc du Cinquantenaire. Head right at the EU headquarters to see Hôtel van Eetvelde before carrying on down to la Grande Place enjoying the views on the way.
Come to think of it I did most of this walk myself a few years back, so it is certainly viable.

I've walked from the Temple of Heaven to the Forbidden City before. But I don't think that I will ever walk to the Summer Palace from either of those two. Seems a little too far to me.

In Seoul:

Chandeokgung Palace Complex
Jongmyo Shrine
Royal Tombs of the Joseon Dynasty

All walking distance (Concerning the tombs and its large number of tombs inscribed, 40 tombs in 18 locations, but there are multiple Joseon Dynasty Tombs scattered in-and-around Seoul)

also

Hwaseong Fortress (only a 30-40 min. Subway ride from Downtown Seoul)

All are fantastic sites