Territory is a fascinating and abstract concept.
A wilderness keep in a game about fighting back the wilderness should absolutely have the wilds as a company. However, keep in mind that:
All companies use the same rules. At the ‘mighty empire’ side of the scale, however, one Company may be composed of many smaller companies, with control flowing from the top downward. For instance, one kingdom can be composed of several domains, and itself be part of an empire. Lords of those domains can apply the potencies of their lands as they see fit... unless ordered otherwise by the king, or by Imperial authority. Because Qualities tire out through use, the timing of actions can become a political football— simply by prioritizing his Influence task before his king’s, a lord improves his odds of success. As long as the king doesn’t get angry.
Thus "The West Marches" are comprised of biomes which have regions which have predators. The level of abstraction necessary depends on the goal of the company.
If the Goal of the Company is "Tame the Marches" (A fun campaign concept, I must say) then you'd want to have a patchwork of regional biomes, perhaps infested with primal spirits because sentient opposition is far more interesting.
However, this doesn't fit very well because it suggests that the Marches are actively resisting being overcome. The best fit is the "Experience" option where:
The final way to permanently improve a Quality is having your PCs take risks and work hard. Your character earns experience points, as described on page 55. You can spend those to improve your character’s Stats or Skills and such, or you can spend them to improve your Company’s Qualities.
You have to justify the increase, but not very much. If you’ve spent several adventures with the army, raising Might is reasonable because the troops know your face. If you’ve been dining with the cultural elite, increasing Influence makes sense. Wiping out smugglers and bandits? Increase Sovereignty or Territory. Anything you do that makes your Company better can find a reflection in a Quality.
Thematically, however, I think the best combination is to model seasons as sentient companies, to reflect the natural anthropormorphizing qualities of humanity. "Autumn helped us gather our crops in, but Winter attacked us savagely." makes a surprising amount of sense as metaphor.
Seasons as companies each have a territory equal to the territory of the company. In order for the company's territory to increase, all four (or maybe 3 of the four) seasons must be defeated.
The Might of a season represents the numbers of inimical forces that that season employs against thinking life. Winter should have the highest might, representing Cold, Famine, and Starving Animals. (This of course, means that the PCs can go off in fights against significant opponents.)
The Treasure of a season primarily represents resources useful to the season like edible products. While iron deposits may be useful to humans, they're not hugely useful to the season. On the other hand, as the player's company gradually fights the seasons for their treasure, the realization of that treasure should be in native natural resources to be exploited. Thus, winter raids for treasure-as-food while the players fight to keep a mine open despite winter-frost.
The Influence of a season represents its inherent discoverability as well as its resources for getting into places sneakily. Generally the seasons will have an influence as a function of their weather and noxious-vermin. Lots of bugs, freezing fogs, downpours: all of these fight scouts trying to learn about the threats of a season. At the same time, bugs can sniff out hidden Treasure (as food).
The Soverignty of a season represents its "sheer cussedness." Is spring easily tamed, or is it a creature of flash floods, unexpected frosts, and bears?
The territory of a season always starts out equal to the territory of the company, as it represents the current lands that the company is trying to hold and secure. Reductions in territory represent capital improvements the company puts in to mitigate the influences of any given aspect of nature. Once all X seasons have been reduced as per company conflict rules, the players can choose to expand their territory by one and begin anew, with a larger land.