Non-exhaustive list: Anything to do with plumbing, electrical, networking and audio cabling. Essentially if you ever think that you may one day want an in home audio system, security system, LAN, it is cheap as hell to put the cables in the wall now and just not terminate the ends. I also put some empty PVC in the walls in case I ever want to run any other cabling, it'll be easier to fish the line. FWIW, I put all my own cat6 cabling in and didn't have the electrician do it. 2 cables to all rooms, 4 to my office, 4 to the entertainment center. Plus cables to all four corner roof soffits if I want to put in security cameras.

I also took pictures of everything before the drywall went up so if I have to get into the walls, I know where electrical and plumbing sits.

We didn't finish our basement because we can always spend more money later, it is harder/more expensive to build more space. So we maximized how big the house was at the expense of not having a finished basement.

However, we did finish the stairwell down to the basement and didn't hide that stairwell with a door and wall. It is all open with railing and carpet and lighting. It really opens up the first floor and looks really nice.

We also plumbed the basement for a bathroom. There is nothing there, but the pipes are there sitting in the concrete foundation. (again, much cheaper to put the $50 worth of PVC in the foundation when it is poured instead of using a jackhammer when you finish the basement.

If you want any built in shelving, it is a lot cheaper to have it done while the trim guy is already there instead of waiting until later.

We spent an extra couple grand on spray foam insulation, my heating/cooling bills are about 30% lower than my neighbors .

That is all I can think of right now.

Think infrastructure: putting in quality wiring/plumbing/hvac is much cheaper on new construction.