“Blending over the seams” between modules is a phrase I picked up from one of Lazy Litch’s ads; the name for it I mean. The actually-doing-it part (as opposed to the knowing-what-to-call-it part), I’ve been doing from day one.
It means making sure the modules you use run together and bleed into each other.
Having similar NPCs between two modules actually be the same person, or having clues to one module be in the next module etc. And replacing cultures so that a reference to “Thorcin” can be swapped out to a reference to “al-Badia” if that better fits the world you’re running in, or ice can become sand.
I don’t like doing this too much, because there is some joy for the players to know that “Oh, OK, we’re ‘doing Arden Vul’ now” or “Aha, we had a good time at the ‘Isle of Dread’” so they can have a shared experience with other groups all over the world, and the other drawback is that everything becomes the same, if there’s only ever one singular cult of evil mages, that might get a li’l bit samey to keep fighting over and over again as if it were the Foot Clan and we were on NES, but I still do it plenty enough. It also helps make the modules even less railroady, even more unpredictable.