GRAMPS offers several basemap options for the map view, including OpenStreetMap. A basemap of the present certainly helps me orient myself on the map, but it can be misleading in some ways. For example, both sides of my family immigrated from South Vietnam, a country that no longer exists. Before that, they migrated from villages in northern Vietnam that were successively transferred to different provinces and later swallowed up by larger cities. It’s harder to get a sense of my ancestors’ background just by looking at OSM or even a satellite map. Even the hospital where I was born was later destroyed by a hurricane, demolished, and replaced by a Walmart store.
OSM has a sister project, OpenHistoricalMap, that seeks to map the world in detail throughout history. Although this ambitious project is still in its infancy, it has extensive and growing coverage of historical boundaries, especially in Europe, the United States, and East Asia. Like OSM, it welcomes contributions from the public, so that a user can map the local context around any events they want to display for their family tree: the church, no longer standing, where a couple was married; the shop that someone owned that later closed due to the pandemic; the office where someone worked that moved after a merger.
OHM would be a fantastic addition to the map view, in my opinion. Perhaps not as the default option, since the database and cartography are still such works in progress, but at least as an option for users to try out, in case they’re lucky enough to have local coverage or are willing to roll up their sleeves and map it themselves.
OHM would be more challenging to integrate than the existing basemap layers. Since it isn’t feasible to render a separate raster tileset for each year in history, the project only publishes a vector tileset that needs to be rendered client-side by a library such as MapLibre (maplibre.org) or libshumate. It also requires client-side code to filter the data to just the relevant dates. The “OpenHistoricalMap/Reuse” article on the OSM Wiki provides an overview of existing services and packages for using OHM outside of the main website.
As a contributor and advisor to OHM, I’d be happy to help this community figure out what’s needed to successfully integrate OHM into GRAMPS. As this might also be the first time some in the genealogical community have heard of OHM, I’d be happy to provide more information about the project. I’m sure OHM also has plenty to learn from this community about best practices in historical research.
In either case, it won’t be a straightforward as dropping in an additional tileset URL template. OHM tiles are in MVT format, not an image format. Each tile contains data about every year, so it’s up to the client to filter it down to what’s interesting – that could potentially be a new UI feature. There’s 
even if it’s still in an early stage. So I’ll be very happy to support what’s necessary to integrate that into Gramps Web. Any help (also for how to generally enable vector tile support) would be very welcome.