Gate crossovers are this strange subgenre magnet. You see the usual suspects all the time: the 'Modern Military versus Fantasy World' setup is basically catnip for certain writers. Rory Mercury shows up constantly, obviously, because she's designed to be fan-service-y and overpowered. But I've noticed Itami gets sidelined a lot; the stories that blow up are rarely about him. They're more about dropping another universe's military or characters into that portal.
Honestly, I got tired of seeing the JSDF replaced with, like, the UNSC from 'Halo' or the Imperium of Man. It's a template at this point. More interesting are the weirder mashups I've stumbled on—one had the cast of 'Stardew Valley' just trying to farm in the Special Region, which was bizarrely peaceful. Another crossed with 'Overlord', making Ainz the one coming through the gate. Those stick with me more than the hundredth retread of curbstomp battle scenes.