Awesome idea! I was thinking about something like this too for a long time, though it's currently in a mental box labeled "probably not in this life" for me. As others in this thread mentioned, there are quite a few interesting challenges that would probably need some experiments how to resolve in the future, though I totally do think "starting small" from a somewhat simplified prototype, and then hopefully iterating, is most certainly the only possible way to do this. Summarizing/expanding on some challenges I've seen noted by others in this discussion, and adding "a few" of mine that I randomly pondered sometimes (while not even being a history buff):

• although (IIRC according to something I've read recently) land surveying was practised in established "countries" since at least babylonian times, I believe there's quite a few challenges to the notion of "firm and precise borders"; going kinda back in time, this can include:

- access to precise survey data might not be easy even in modern day, with this getting harder and harder + lines more and more blurry the further we go into the past;

- as noted by others, some borders and territory ownership is heavily disputed even in the modern day;

- during wars, this gets even more unclear kinda by definition (a lot of "we control this territory" can change as frequently as many times a day, and is anyway often vaguely somewhere on a spectrum between "fact" and "wishful thinking"); [as a side note, I was kinda thinking it would be even more awesomesauce to show army movements and other population data on such a historical map as well, however this adds a whole new universe of complexity];

- a notable subcase of the above points being "deep state" situations, where the official country's officials might have a varying (sometimes tiny or none at all) authority over some parts of their territory, controlled e.g. by organised crime;

- in less "organised" countries (incl. more "primitive/aboriginal" ones), I suspect land survey might sometimes not exist at all; and/or, it may be based on some rather different cultural notions of "ownership" and "borders";

- in fact, AFAIU, the notion of "countries" is kinda something that is not a given at all historically; I believe throughout history something resembling a feudal model (or a kingdom) is much more prevalent, i.e. "I [a king] claim to be owner of this land", where I would imagine it's much too easy to make such claims; and then, with all what I wrote above, even looking at "de facto" ownership and fairly "organised" kingdoms, based on just some pop-historical literature I believe especially the closer you got to a "border" between the kingdoms, the more it resembled permanent "war territory", with "quick border raids" etc. and local skirmishes being so obvious, that nobody could even imagine it might look different in the 20th+ century (with things like The Chinese Wall and Hadrian's Wall being kinda monuments to that reality); for another possibly nonobvious organisation model from modern point of view, see ancient Greek city-states;

• time tracking being very different (e.g. "king X's Nth year of rule") and sometimes tricky to map to a universal timeline, such as the "AD/BC" (a.k.a. "CE/BCE") scheme (e.g. can we pinpoint the start of "king X"'s rule based on available sources? was it even a real king or is it purely a mythical figure? do all the sources claim the same X and N?);

• as others wrote, natural land borders & shapes definitely moved & changed over time; this can include but is not limited to:

- rivers (see other comments); artificial islands; volcano islands; volcano "mountains"; dams; floods (incl. permanent floods into land-locked depressions); drying lakes/seas (see e.g. Dead Sea, Caspian Sea);

- ice, incl. the "ice ages"; maybe a stretch case, but IIRC the Baltic Sea is known to have frozen in historical times, to the extent that inn(s) for travelers crossing it over the ice were built on that surface [though "citation needed", i.e. I totally did not fact-check it];

- go far enough in time, and continental drift itself becomes clearly apparent; this means not just broken landmass continuity or new landmass connections, but also new mountains (which often tend to become natural borders).

And that's just off the top of my head ;D I think stuff like this makes such a project super interesting and novel! :)