As the gaps studies as well as more focused thematic studies often lead to inscriptions I thought a summary of such remaining gaps is of interest.
Current T-lists are listed for each gap.

Bioms:

1) Andaman Sea - Myeik Archipelago (Myanmar), Narcondam Island (India)
2) Benguella Current - Benguella marine ecosystem sites (Namibia)
3) Fiji (marine) - none
4) Karoo Desert - Succulent Karoo protected areas (Namibia and South Africa)
5) Maldives-Chagos Atolls - none
6) New Caledonia dry and moist forests - none
7) Red Sea - Ras Mohammed (Egypt)
8) Sudd-Sahelian Savannah and flooded grasslands - Lake Chad (Chad, Cameroon, Niger)
9) Tahiti - none
10) Volga and Lena river deltas - none

Habitats:

11) Eastern Arc Mountains (Tanzania)
12) Atlas Mountains - Ajgal (Morocco), Talassemtane (Morocco)
13) East African mangroves - Jozani (Tanzania), Quirimbas (Mozambique)
14) Kalahari (Botswana)
15) Somali-Maasai savanna - none
16) Subtropical forests of Cambodia - none
17) Tropical montane forests of Sulawesi - Bunaken (Indonesia)
18) Temperate deserts of Central Asia - Amu Darya (Turkmenistan), Tigrovaya Balka (Tajikistan)
19) Shrubland of the Southern Caucasus - Colchis Forest (Georgia)
20) Rainforests of Polynesia and Micronesia - Sovi (Fiji), Fagaloa (Samoa)
21) Mangroves of Eastern Papua and Northern Australia - Upper Sepik (PNG), Kikori (PNG), Transfly Complex (PNG)
22) Californian shrub - none
23) Central Mexico Desert - Big Bend NP (USA)
24) Rainforests of Southern Chile - Los Alerces (Argentina)
25) Mosquito Coast - none
26) Temperate forests and shrubland of Central Chile - none
27) Grasslands of Falklands and Tierra del Fuego - none
28) Tropical Andes Shrub - none
29) Coastal deserts of Chile and Peru - none
30) South Georgia - none
31) Montado - Monfrague (Spain)
32) Maquis - Sila (Italy), Arrabida (Portugal), Port-Cros (France), Samaria (Greece)

TBC

http://whc.unesco.org/en/list/1115/?multiple=1&unique_number=1531&
If we look at the official map, the terrestrial ecosystems of New Caledonia are indirectly represented. But it is true that New Caledonia, with its exceptional endemism, deserves to have a second natural property inscribed (Parc provincial de la Rivière Bleue...), more focused on the terrestrial aspects. I visited the island twice, soon 3 times as I have family on site, I can confirm that it is a botanical paradise.

Deserts:

1) Western Desert Egypt - Great desert landscapes, mountain chains, southern and smaller oases (all Egypt)
2) The Chotts - Chott El Jerid (Tunisia)
3) Thar Desert - Desert NP (India)
4) Band-e-Amir (Afghanistan)
5) Las Parinas - none
6) Atacama - none
7) Gobi (China and Mongolia)
8) Wadi Howar (Sudan)
9) Sabkhas - Bar al Hakman (Oman), Boubyan (Kuwait), Khor al Adaid (Qatar)
10) Badain Jaran - none
11) Arches - none
12) Colorado Plateau - Petrified Forest NP
13) Death Valley - none
14) Lake Bonneville - none
15) Bodele Depression - none
16) Kimberley Mts - none
17) Hunza Valley - none

Marine:

1) Scotia Sea - none
2) Agulhas - none
3) Easter Island (marine) - none
4) Gulf of Guinea - none
5) Humboldt Current - none
6) Northwest Atlantic - Jan Mayen (Norway)

Volcanic:

1) Fissure volcanoes - extension of Thingvellir NP (Iceland), Myvatn-Laxa (Iceland)
2) Hyaloclastic mountains (tuyas) and ridges - Thingvellir extension (Iceland)
3) Icelandic type shield volcano- Vatnajoekull (Iceland)
4) Continental Flood Basalts- none
5) Resurgent calderas- none
6) Large ash flows and ignimbrite sheets/plateaus - none
7) Intrusive landforms- none
8) Basaltic plains - Vatnajoekull (Iceland)
9)Mt St Helens and Crater Lake - none
10) Mt Pelee - Aires volcanique de Martinique (France)
11) Laki - none
12) Mt Mayon (Philippines)
13) Thera - none

Karst:

1) Canyon du Rio Peruacu (Brazil),
2) Gewihaba - none
3) Velebit Mountain (Croatia)
4) Svalbard (Norway)

IUCN has released a new report of 20 amazing African sites that are priority for World Heritage Sites.

1. Bale Mountains National Park - Ethiopia
2. Benguela Current Marine Sites - Namibia
3. Bijagos Archipelago - Guinea Bissau
4. Boma - Badinglio Migratory Landscape - South Sudan
5. Chott el Jerid - Tunisia
6. Dinosaur Fossil Bed of Niger - Niger
7. East African Coastal Forests - Kenya / Tanzania
8. Eastern Arc Mountains - Kenya / Tanzania
9. Equatorial Atlantic Coastal Forests - Congo / Gabon
10. Erte Arle and the Danakil Depression - Ethiopia
11. Great Western Desert - Egypt
12. Lake Tanganyiga - Burundi / DR Congo / Tanzania / Zambia
13. Madagascar Dry Forests - Madagascar
14. Montane Forests of the Southern Albertine Rift - Burundi / DR Congo / Rwanda
15. Mozambique Channel - Comoros / France / Madagascar / Mozambique / Tanzania
16. Protected Forests of the Gulf of Guinea - Cameroon / Niger
17. Succulent Karoo - Namibia / South Africa
18. Sudd Wetland - South Sudan
19. Tropical Moist Forests of the Upper Guinea Biodiversity Hotspot - Ivory Coast / Ghana / Guinea / Liberia / Sierra Leone
20. Volcanic Islands and Forests of the Gulf of Guinea and Cameroon Highlands - Cameroon / Equatorial Guinea / Nigeria / Sao Tome and Principe

https://portals.iucn.org/library/node/49029