Well, one notion from Dresden is that areas of similar concepts are where portals can connect. So a shady alley where people are killed often would usually connect to a more evil place in the world. That could help.

~SE++

That doesnt really address the technology shift though.

It's written by the author of the Animorphs but aiming for older teens so it's ~edgier~. The idea is that the ancient gods (even though they actually really cover thousands of years rather than being contemporaries, from Ancient Egyptian to Classical Greek to Medieval Nordic to European Renaissance-Era Mesoamerican) left Earth and created this patchwork world called Everworld. Teens from Earth get pulled there with a lot of consequences, including one of the characters teaching fairies how to build the telegraph.

I was just thinking the other days "You know, I've never seen a fairy that knew how to use a telegraph"