underground pipping

what about underground pipes? dig first with excavators and than put the pipes down ^^

I think most underground pipes aren't as deep as the game stimulates right now. It also sounds like a hassle to trench and refill so much ground.
That's not to say it's a bad idea, just that it might be a situation where you need a digger assigned to build these pipes, but it doesn't actually change the terrain.

might be a good idea to add a new Trench Excavator machine to lay underground pipes
https://blog.iseekplant.com.au/hs-fs/hubfs/best-trencher-brands.jpg

Good idea, as an extra option to tidy up your island and avoid truck pathing issues. And I strongly agree that it should come with increased cost (in effort). Some form of (automated) trench digging should be required, forcing you to really commit to a pipe layout. Otherwise, if it is just as easy to pipe underground, it will become the obvious default for every connection, and that would take away from the game's charm of having visible pipe spaghetti in some places.

you can do this already just dig a hole put pipes in the hole then fill the hole done and it still works but not for conveyors

Underground pipes look neat but would be much easier to build and upgrade. A special vehicle for that would be nice to place with tier II being able to palace deeper.