The Caucasus population is very likely the genetically most diverse population worldwide thanks to the intermixture with A. bison followed by absorptive breeding; as outlined in the post above, the high inbreeding of the two "pure" bloodlines has severe effects on some populations, including drastically lowered resistance against certain diseases . It is likely that these problems are less severe in the Caucasus population, maybe even to a considerable extent. Unfortunately no study on that has been carried out yet, probably because of the a priori lack of interest for this herd. Furthermore, the Caucasus population is the only Wisent herd today that has been living in the wild for the last 70 years, so it probably were always the healthier and hardier animals that survived and reproduced, and if there were disadvantageous features inherited from the American bison they probably disappeared. So to me, the Caucasus population is worth to be preserved, protected and also expanded - but this expansion should happen by further mixture with "pure" wisents, or, if their number increased sufficiently, selective culling of those wisents with optical features of their american relatives. There is a neighboring herd of LC-wisents and I think both populations should be merged together.
In the 1980s, the Caucasus herd was the only large and vital population of the wisent since the 19th century counting incredible 2.300 animals . Poaching dramatically reduced them down to about 200 animals. Not only this itself is a shame, but also that the disinterest of conservation on this population might have been one of the reasons why poaching had such a severe effect. Thankfully, the Naturschutzbund Deutschland (NABU) in cooperation with Russian conservation groups now protects this herd by supplementary feeding and rangers. If you want to support their efforts to conserve the most genetically diverse and vital and sadly most underrated wisent population in the world, go to this website and scroll down where you can donate for this project.
This plea is not only for the Caucasus herd. There are about 700 Wisents which are excluded of the breeding book and therefore "not existing" just because they have no documented genealogy - this does not necessarily mean they have hybrid ancestors, it is also possible that some of them are "pure" wisents that even represent bloodlines of the 12 ancestral animals that are now lost in the gene pool formed by the animals of the breeding book. And even if those animals have bison or cattle hybrids in their ancestry, I do not see that as a reason to exclude them from conservation completely. As you might know, a large number of American bison are genetically influenced by domestic cattle. Although the American bison has a large, genetically diverse global population, seemingly nobody had the idea to start a similar pedigree cult in this species, although it probably would not cause severe inbreeding problems, contrary to the case of the Wisent. And as a side note, there is almost no optical indication of a hybrid ancestry in many of those bisons, so why should that be the case in the wisent if the breeding with such animals is carried out carefully?
So my proposal for reducing the threat of the Wisent failing repopulating the wild because of a lack of genetic diversity is:
- Opening the lowland line for introgression of the lowland-caucasian line and closing the latter from introgression, until the inbreeding coefficient is equal in both; after that, both lines should be merged together
- creating another blood line group that consists of the wisents without proven "pedigree", also intermixed with animals from the "pure" bloodlines
Of course "pure" Wisents should be preserved and at least one closed gene pool should consist of these animals exclusively (I see no point in separating the LC and L line because of the little differences between the former two subspecies). But what if many of them are deformed, infertile or prone to diseases and parasites so that no viable population below 100 individuals can be set up? I think that genetically diverse populations consisting of the blood group I propose above are necessary for creating more vital wild wisent populations.
 Mammal Research Institute, Polish Academy of Science: European Bison Bison bonasus: Current state of the species and an action plan for its conservation. 2002 http://www.derwesten.de/staedte/nachrichten-aus-meschede-eslohe-bestwig-und-schmallenberg/waldbesitzer-aergern-sich-ueber-wisent-schaeden-id8568518.html [German]
 Bunzel-Drüke, Finck, Kämmer, Luick, Reisinger, Riecken, Riedl, Scharf & Zimball: Wilde Weiden: Praxisleitfaden für Ganzjahresbeweidung in Naturschutz und Landschaftsentwicklung. Arbeitsgemeinschaft Biologischer Umweltschutz