Interesting idea. I'm sure we would all like to be able to claim more territory. I think there are some important balance considerations the devs have to make when answering the question "how much territory should a guild be able to claim?" and I'm not sure how well this would play with their answer. But expanding it in particular areas via an additional structure definitely seems like an interesting alternative to just giving the monument another higher level.
I don't know if they will add higher levels to the base monuments but even in LiF:YO there is the option to build "Border Monuments" in either the construction or masonry skill trees at 100, but it is disabled due to the map being so small.