
When playing solo, extra seats are completely unnecessary. I think that when you unlock this chassis in the base builder, additional items like a seat, storage, or fabricator should unlock, and the chassis should be created with empty slots where you can add two storage or seats, or one storage and one seat, depending on your needs.
Maybe you can choose of these features when creating the chassis or if you could add modifications

This could even it out between solo and co-op worlds since the player sitting has an inventory, so co-op worlds would be able to hold more storage in the chassis. So it would make sense to replace that player/seat with a storage spot. Maybe even replacing the connection point for portable storage.

it would be also great, if the haul chassis could… haul things: https://subnautica2.nolt.io/12409

Great idea to be able to customize your haul chassis with the Builder tool.
This thing currently isnt very usefull for singleplayer or multiplayer with low numbers. I like this approach
This would be amazing!
Though I would request that instead of making a whole “complete” chassi, why not make a Chassi Frame that has Modular Interfaces like the one on the back where you can attach a Tool or a Floating Storade, so then you can Equip/Attach or Unequip/Dettach what you want, when you want. You just need to craft the Modules
As for the fabricator, I’d be down for that, but only a very basic fabricator with limited recipes, like make water and food and may a few more.

I made my own post awhile ago with this kind of thing in it: https://subnautica2.nolt.io/5629

Or even add extra modules for speed
Like auxiliary engines

Why not just fabricator add on that can attach to one of the hard points?