electricity related props, like telephone poles, lamposts and electric pylon

a bagel.

possibly, if this is for the modular sewer mod, as thats were i found this at, some bigger versions of the sewer parts, and a dropdown part to go deeper like you can see in some movies like ratatouille? to make it were you can do more with it in general also so you can escape him easier? note i havent touched the mod yet, was just going off the screenshots for the current parts so i though i would say it in just in case.

Perhaps some shorter endcaps, or a stretch with an (unusable) "exit" like a service door or ladder?

6

fireworks

how about a version of the wall-less one with just one wall? or a tunnel with (room for) a hatch at the top?

a tunnel with a bridge over your head

modular cave system

Heavily suggest adding compatability with track assembly.

idk like gas pipes or water pipes

interior parts

cave system like @gudderpunk suggested