In my diary of the trek, it says that we crossed the park entrance after 3 hours on day two (after having arrived in Lukla by plane on day 1 at 10.15h). So it's well within 5 days. Plus: nowadays it's possible to get to Everest Base Camp by helicopter from Kathmandu Airport (http://www.getyourguide.com/kathmandu-l101/everest-base-camp-helicopter-tour-in-nepa l-t25515/)

Garamba should be possible within 5 days: http://www.ventureco-worldwide.com/garamba-national-park/
Some even do it by bus: https://www.lonelyplanet.com/thorntree/forums/africa/congo-the-democratic-republic-of /garamba-national-park-thoughts-advice

I also now have added the 'In Danger' and 'takes more than 5 days to visit' rankings to the statistics page:
http://www.worldheritagesite.org/stats.html

Never mind Durian - there are some (surprising?) positives (visa wise!!) to having Chinese Nationality.
I would very much like to go back to Iran where I have only visited 5 of its 19 sites (way back in 1971 so no "WHS to "target" in those days ).
I recently arranged an itinerary with an Iranian travel company and they said that I would need to book quickly as all the hotels were getting filled up with tourists! Really - Iran isn't known as a "busy" tourist destination?? It turns out that a large number of Chinese are visiting!!
Indeed they can have "Visa on Arrival".
On the other hand, since the UK and Iran have no diplomatic relations my UK passport condemned me to have to travel IN PERSON to Dublin, having previously booked/paid for my flights, to have my finger prints taken, beard length measured etc - with NO guarantee that a visa would be issued (I decided not to risk it and hope that UK/US can patch things up with Iran in the near future since both are opposed to Sunni extremism - "A" bomb or no "A" bomb!)
Chinese can also visit Cote d'Ivoire and Burundi on an e-Visa - so get travelling!!

My first connection contribution ;)

To add some more to that: Assuming difficult sites are less visited I checked for each site with less than 2 visitors how difficult it would be to get there. I also took a look at the map and added those sites with nothing nearby... Starting with Africa. I do assume we are not flying around in private jets.

Feasible in < 2 days per trip:
* Air and Ténéré Natural Reserves, Niger: Day drip from Agadez (WHS, 100km). Unsafe.
* Okapi, Congo: Virunga is 250km. Should be feasible in 8h. Probably unsafe.
* KAHUZI-BIEGA NATIONAL PARK: Clustered with Virunga, Okapi, ...
* Socotra (Yemen): Can be done from Sanaa by air plane, so should take less than 5 days.

Candidates:
* Sukur, Nigeria: Can't find any travel tips for this one. Looks like you need to get to Yola and from there you would be able to get there. No sites in proximity. Could take more than a day back and forth. Unsafe.
* Sangha Trinational, Cameroon: Dja Reserve should be two days travel from/to here.
* Dja: You need to get to Yaounde first. From there at least a day's travel. Next site (Sangha) probably also two days away.
* Garamba, Congo: Either from Okapi or from Juba this will probably take more than a day travelling. Also, South Sudan definitely not safe.
* Lakes of OUNIANGA, Tchad. In the middle of the Sahara. No sites at all in the proximity. 4x4 from N'Djamena. Unsafe.
* Salonga, Kongo: Major cities and Okapi both > 50km away. I think 2 days travel time each way is safe to assume.
* Manovo-Gounda St. Floris National Park. No site in proximity. No airport either. 2 days travel effort per direction seems reasonable.
* ALDABRA ATOLL (Seychelles): Pretty clear candidate simply reading Paul's review.
* Vallée de Mai (Seychelles): To get in and out of the place will probably take two days each.

Different point of view: Sites requiring more than a day to fully appreciate.

This can be done from Rwanda in a 4 day returntrip. I actually looked into it preparing for my Rwanda/Uganda trip next December.

http://www.rwandagorillassafari.com/congo/4-days-kahuzi-biega-lowland-gorillas.html

Also have looked recently into getting to Sangha Lodge, probably the most attractive destination within Sangha Trinational WHS.
Can be done in one very long or two days from Bangui.

http://www.sanghalodge.com/pages/getting-there.php

You'd want to stay longer of course, but it is doable within 5 days return.

I was actually counting sites requiring five days and more. Not more than five days ;) I think every site that requires two days of travel to get there and then back again is special.

You started counting with this, which I think is fairly accurate:
* 2 WHS / day: 200 sites globally = 100 Days
* 1 WHS / day: 300 sites globally = 300 Days
* 0.5 WHS / day: 300 sites globally = 150 Days
* 0.33 WHS / day: 200 sites globally = 600 Days
* 0.2 WHS / day: 31 sites globally = 155 Days (e.g. those australian islands)

However, we've now found at least 10 WHS that take even more time than your toughest batch, ca. 0.03-0.1 WHS per day!

This was just a back of the envelope calculation what it would take. ;) And I agree, 5 years probably still doesn't cover it.

Just did my private calculation today and noticed that there are no WHS in a 500km radius left.

Thank you all for the input

Using the current rate of nominations. 26.44 per year (+/-)

Current Tentative List: 1630
Approximate Indicative List: 200 (+/-)

In theory the current Tentative List and "Aspiring" nominations will be exhausted by 2084 (26.44 inscriptions per year)
Low Estimate by 2084 - 1,943 World Heritage Sites (13.22 inscriptions per year) 50% cut in inscriptions
High Estimate by 2084 - 2,861 World Heritage Sites (26.44 inscriptions per year)

In 2084, I'll be the grand old age of 99. Based on our current tentative lists and forthcoming nominations in the coming years + the current average of inscriptions per year, I have some reservations about ever completing the UNESCO World Heritage List!!!

Things to consider
1) In all likelihood, sometime in the 2040's or 2050's the first WHS with its OUV connected explicitly to the 21st century will be inscribed.
2) Many of us will see the 2,000 world heritage site in our lifetime.

Nice story on Jorge Sanchez, completionist par excellence:

http://www.bbc.com/travel/story/20161209-the-runaway-who-travelled-the-world?ocid=fbt vl

I recall he once had a profile here with many interesting reviews. What a pity they are gone.