I don’t know if this is the right place to ask about mainlined mod content, but it would be really cool to be able to make an amphibious vehicle with the boats mod. Maybe add a “Switch Control Scheme” menu to vehicles that would work like “Switch Engines” where you swap out wheels for boat boards once you’re in water? That way you couldn’t go into a river from land without slowing down, and also it’s just fun to imagine my deathmobile wheels folding inwards, pontoons inflating, and my survivor escaping to open waters. As it is, you can still cross a river with a vehicle, you just need to extend the frame across the river, put a wheel on the end, and rip the whole apparatus apart on the other side. It’s hardly immersive and incredibly tedious to do. This could also be used if you wanted one set of wheels that are good on roads but wanted to swap to a set that’s better offroad.
Separate idea, but slightly related: deployable rams where the logic is they raise like in real life, excavation arms to collect large amounts of rubble/dig large pits/pick up river tiles (this wouldn’t deplete the river tile, I’m assuming rivers are flowing and you’re not taking enough water to affect the current), dump trucks that could hold rubble/dirt/liquid (at very low speeds of course). Depending on where they are placed in relation to the vehicle controls would determine which cardinal direction they deploy at, making multi tile parts like the dump truck move contents smoothly with tiles closer to the controls moving to the next one over until it ends up on the ground/in another vehicle. You could rapidly build dirt banks to work as improv walls but can wash away unless you plant grass on 'em, dig a ditch and place a water tile in it to make a moat, or scatter rubble strategically on the tiles surrounding your base. The construction parts could be prohibitively heavy so you can’t just throw them on a deathmobile, it would have to be it’s own dedicated vehicle. They could be incredibly rare and nearly impossible to craft, and you could add a couple of very rare construction sites to worldgen, one small one in cities, one as a multi map tile next to a road.
I think I know how it could work with z levels: if you dig in one spot long enough, it makes the tile impassable on foot but vehicles can go on it (so you an hollow out large swathes with an excavator), call it a deep pit or something. You then either put water in it to get deep water tiles (maybe need to pour concrete to prevent seepage) or build a “foundation” on the tile, which you can cover with dirt to make it normal ground or put wood on it or pour concrete or whatever to make various types of floors. When the process is complete it hollows out the tile under the one you were working on, thus “moving” the empty space down one z level. The code to affect lower tiles is already in place, just cannibalize the “dig basement” construction code.