Maybe not top 50 but still - we know so little about previous cultures of this enormous country. The following link http://www.wondermondo.com/Russia.htm supplies this list of archaeological sites in Russia:

One of the most impressive fortifications in the area of Russia is Narin-Kala fortress in Derbent. It is a 40 km long Sassanid fortress from the 6th century. A unique monument is Por-Bazhyn, an enormous quadrangular fortress on an island in Tere-Hol Lake in the Tyva Republic.
Numerous ancient cultures developed large settlements, but unfortunately most are lost today. Some remains of the ancient Volga Bulgaria capital, Bolghar and Bilar are still visible in Tatarstan. Many Russians see the ancient Arkaim settlement (17th century BC, Sintashta-Petrovka culture) as a monument with an aura of mystery.
In the Russian steppes there are numerous ancient burial hills - kurgans, which can be found from the Amur region in the east to the countries west of Russia. These hills were created by numerous diverse cultures over many thousands of years, from the 4th millenium BC up to the 10th century AD. In spite of looting, lots of valuable (scientifically and monetarily) artefacts can still be found in these burials. Outstanding kurgans are Pazyryk kurgans in Altaii region (5th c. BC) where numerous priceless artefacts have been found and the once 27 metre high Great Salbyk kurgan, with a 496 metre circumference, in Khakassia.
Other very interesting monuments are the diverse megaliths and petroforms of Russia. Still not well known are the more than 3000 unique dolmens of north-west Caucasus, such as the Zhane River dolmens. There are several dolmens on Vera island (circa 4000 BC), Turgoiak Lake in the Southern Urals. Numerous interesting monuments are located along the White Sea and the Barents Sea, such as stone labyrinths of Bolshoi Zayatsky island.
Closely connected with kurgans are stone idols, which stood on the tops of the kurgans. Such sculpted stones, 1 to 4 metres high are found on numerous sites, such as the Baal-Bab valley idols and the Inya stone idols in Altaii.
Numerous cliffs bear lots of ancient petroglyphs, such as Boiarskaja pisanica in Khakassia, Kanozero petroglyphs (2000-3000 BC) in Kola peninsula, Kalbakh-Tash in Altaii, Pichiktig-Tag in Khakassia and the unique petroglyphs of Ignateva Cave in Chelyabinsk Oblast as well as the beautiful cave art of Kapova Cave in the Republic of Bashkortostan.

Of these Derbent is already on the list. Sikachi Alyan petroglyphs, Tanais and Bolgar are archaeological sites on the T list not mentioned here. What about the rest of this variety of archaeological heritage? Why are further nominations of these sites not currently pursued? Any thoughts?

My imagination has long been "enthused" by the "Mammoth Bone dwellings" of Central Europe from c 15000 years ago ("some of the oldest shelters known to have been constructed by pre-historic man") Mainly Ukraine but also Russia and Moldova so still relevant to your subject I feel in representing a type of site significantly "hidden" from Western knowledge/access over the years and hence under-valued. Geographically they are located on the "Russian Plain". The most famous is at Mezhirich Ukraine (But see also - Mezin, Gontzy and Yudinovo - the latter is in Russia). This Web site has some photos of the excavated site, reconstructions etc at Mezhirich
http://donsmaps.com/mammothcamp.html

Another significant type of archaeological site from the area is that of "Mesolithic Burial Grounds" from 10th to 7th millennium BC e.g Oleneostrovski Mogilnik in Russian Karelia and Vasilievka in Ukraine. These demonstrate the development of society in post glacial Europe
http://deepblue.lib.umich.edu/bitstream/handle/2027.42/24870/0000297.pdf?sequence=1





Neither type is represented as far as I can see on any T List?? Nor do they figure in the list of Archaeological sites in the wondermondo link?

I think there is certainly enough material here for future WHSs. Solivagant's mammoth bone dwelling indeed look inspiring. I don't know why such sites are so often overlooked. Russia (and partially the Ukraine) could certainly propose some interesting archaeological sites mostly unknown to the Western world.
I'll list some of the suggestions:
1) Solivagant's mammoth bone dwelling in Mezhirich (Ukraine) http://en.wikipedia.org/wiki/Mezhyrich
2) Oleneostrovski Mogilnik
3) Por-Bazhyn http://en.wikipedia.org/wiki/Por-Bazhyn
4) Arkaim http://en.wikipedia.org/wiki/Arkaim
5) Atil - Samosdelka http://en.wikipedia.org/wiki/Samosdelka
6) Pazyryk burials http://en.wikipedia.org/wiki/Pazyryk_burials
7) Zhane River dolmens http://en.wikipedia.org/wiki/Dolmens_of_North_Caucasus
8) Vera Island http://en.wikipedia.org/wiki/Megaliths_in_the_Urals
9) Baal Bab Valley http://www.gala-studio.ru/foto.php?sid=50&aid=65&lang=eng
10) Kanozero petroglyphs http://en.wikipedia.org/wiki/Kanozero_Petroglyphs
11) Sulekskie petroglyphs http://www.sayanring.com/glossary/sulekskie_petroglyphs_eng/
12) Ignateva and Kapova caves http://de.wikipedia.org/wiki/H%C3%B6hle_von_Kapowa

Balhae Kingdom Excavations - These excavations in the Primorski Krai region have been ongoing for much of the last decade. Most of the structures found have been ruins of palaces, fortresses, tombs, castles, and homes (including ondal under-floor heating). Most of these links are South Korean sources. The excavations have been a joint operation by Russia and South Korea. The Balhae Kingdom encompassed large areas of North Korea, Manchuria, and Primorski Krai (Russia). There is some of the inevitable controversy between China and Korea in relation to who "owns" the Balhae heritage, Russia has taken a more objective approach.

*Kraskino Fortress
URL, URL

*Chernyatino burial ground, 118 Balhae tombs
URL

*Yeonhaeju Castle
URL

*Ondol Heating System
URL

*Balhae Palace
URL, URL

Found this new article about Russia is taking measures on increasing the number of the country's properties on the list of UNESCO World Heritage Sites by Russian Foreign Minister Sergey Lavrov.

Interesting from the news he mention sites that are underway which are:

1) Sviyazhsk (will be submit on Feb 2016)
2) Astrakhan, Rostov, Pskov and Uglich Kremlins - the idea may combine the refer Russian Kremlin with Rostov Kremlin
3) Valaamsk Archipelago (Valamo)
4) Divnogorye in the Voronezh region
5) Kenozersk Park in the Arkhangelsk region,

http://tass.ru/en/non-political/792195

Another Update on Russia plan for WHS.

Russia wants to propose the architectural monuments of Pskov (400 miles northwest of Moscow) and Sviyazhsk (an island in the Republic of Tatarstan, which is part of the Russian Federation) for inclusion in the UNESCO World Heritage List.

http://rbth.com/arts/history/2015/12/16/russia-to-propose-pskov-and-sviyazhsk-as-unes co-world-heritage-sites_551711

Pskov has very similar architecture to inscribed Novgorod. I think they should concentrate on extending Novgorod by including Pskov. However, I think Pskov deserves thr right to be inscribed. This is probably not the case for Sviyazhsk, which seems to be the next lobbying site for Tatarstan authorities which are vert enthusiastic about inscribing local sites (see Bolgar's case).