No modular rivers and roads are quite common but they have to be modular as they cannot be done as separate items but that seems to be it. Additionally they are not sized or set up to integrate with hills as in the main (there are some recent exceptions) not able to drape over any sort of hill: not are they sized to integrate with a hill system. Hexon II does have some integration possibilities but there system does not cover their slopes.
I guess you could call buildings with detachable levels modular but I would not, as usually they only build into one fixed configuration. Buildings that can be grouped in to different floor plans seem very rare, I have a few (sort of) but even for me they are not that common. I guess in general my approach to building size:- limits it extensively.
TMP link
Do many folk have genuinely modular buildings.
Modular terrain board did rise to fame for a while at least in the UK but they died ot after a while as being 2ft square limited possibilities without a massive collection.
In the US there was Geohex in small and large hex versions but that have all but died out from production. I believe there is some manufacture of the large hex but it seems not to have stayed in the mainstream, certainly not in the UK
There has been some attempts to bring in 3D print systems but again and i am happy to be corrected have not become popular.
Individual hills still seem to be the thing and they are not modular they are individual items.
The only modular system that I am aware of is Kallitra's Hexon II (no commercial connection to me). However I do admit to being a fanatic devotee, having abandoned my own system for theirs. While in the UL it has some following it is not that large, but is a viable serious player. However I am not personally aware of another system of equal popularity.
The rise of the battle mat seems to indicate hills are not a real priority to many gamers. While the odd very low hill may be pushed under a mat, steep hills 2" or higher are an absolute nNo No. The fact that (as far as I am aware) no battle mat supplier does even stand alone hills covered in the style of their mats indicates they are not in demand.
Is that why modular hills are not popular as hills in general are a low priority to many folk?
The dreaded company who shall not be named bu may have the initials GW did a modular terrain hill but it disappeared fairly quickly and was of limited scope.
Just to kill the Sm..t A.. Comments we do provide a limited section of Fold flat hills but they are our worst sellers of all our stuff, despite being modular.