Like the above comment, have your plumbing stubbed out, most importantly have the vent pipes installed and capped in the attic and basement. When doing rough plumbing have T-fittings installed so the future water lines, etc can be easily taped into it. Install under the slab perimeter drain and tie into the sump pit, install under the slab sewer piping or ejector pump if that's what you be using for the future bathroom.

Extra insulation for sound

Plan out the "lower level" (sounds better than "basement"} with as much flexibility as possible before construction starts (hope I am not too late). I read a wine cellar design book once that recommended allowing space for an extra 12 cases of wine just in case a close friend encounters marital problems in the future. Make sure your electrical service will accommodate anything you may want to do with the space. Plan in egress windows if bedrooms may be located there or just common sense safety.

We love our walk our basement! We have a rec room with double French doors leading to the back patio. A guest bedroom and a full bathroom. We also have a one bedroom and den suite on other side of house.

Think about ease of moving furniture into the basement and make sure the top and bottom of the stairs are not in close proximity to a wall.

We also have a big storage area under the stairs which is amazing to have

Not sure what "process of building" means.

But if construction hasn't started, the architect/engineer should design with as few structural constraints as possible in your basement space, i.e., columns and structural beams.

Ditto the mechanical engineer in designing the HVAC system. "9' after putting up the ceiling" is not the same as putting up the ceiling with HVAC returns and supplies in place.

Consider in-floor heating for the basement floor and factor that extra heat in with the HVAC design. IOW, you will need less heat from the furnace or other heating source.

If you want ceilings at least 9 ft, then you need 10 ft walls. 9 ft walls will give you 8.5 ft or so walls. Plan ahead of time for where HVAC vents are going as those will lower the ceilings more. Rough in a bathroom. A future floorplan also helps with all these things.

If you think you ever may ever have a bedroom down there make sure that bedroom area will have an egress window.

If you'll have plumbing on the lower level in the future you will have to locate the rough plumbing now. You should go ahead and lay out a basement finish plan to make sure that plumbing is properly located. The finish plan will help you locate the right kinds of windows and doors on the walk-out level. You can always adjust your plan when the time comes.

You should also pay careful attention to how any decks interact with the lower level. i.e. you won't want a deep deck with lots of heavy columns shading your recreation room.

We didn't build new, but had our basement finished in 2018. In addition to locating the rough plumbing in now for a bathroom, you also may want to think about what else you're going to use the space for. Do you plan on putting a kitchen or kitchenette in, with a sink? Or do you want your own sports pub with a full bar? Where will your mechanical room be? (that may sound obvious but if you also plan on using it for storage, there are clearance issues you need to consider for furnaces and electrical boxes). My husband works from home a lot and we have his modem downstairs in a special dedicated closet - will you need something like that? We also added a big pantry (we call it our Costco pantry) which is one of my favorite things about the basement. As the PP said you can adjust the plan when it's actually time to finish, but having a rough idea of what you want/need ahead of time will really help.