Stay tuned...I'll let you know after my build kicks off in June.
I think most people here don't actually "build"...it's more about conversion of a spare room in the house, but from that perspective, I would say that my biggest change is simply the sheer volume I will add in the new space compared to the current one.
My current space comes out to about 2300 sqft...the new space will be close to 10000 sqft, and that alone will make for much easier room treatment and fine tuning.
In addition to that, I will be able to move my console/mix position much further from the front wall...and I will also change the location of my racked gear...from the sides of the console and then another two off to one side...they will most likely all be directly behind the mix position, about 4-5'...so that when mixing, I can say in the stereo image while reaching back to adjust some rack gear. I always hated having to roll my chair off to the side, make a change, then roll it back to the center so I can check the adjustment...etc.
Finally, I'm going to run most of the audio and electric cabling under the hardwood floor, going from the console and the bays to the gear and to some wall plates at a few locations. That way it will be uncluttered, though TBH, right now if you walked into my smaller studio, you would hard pressed to see a single cable anywhere...they are all behind the gear...but it's a PITA getting at anything because it's all stacked using every available inch of free space.
So I think the biggest thing for me will be the extra space...not having everything crammed up (I've accumulated too many guitars and amps)...but also the bigger space will really be beneficial to the acoustics, which are just not easy to deal with in smaller rooms like most of us have to make do with.
I'm anticipating strategic but minimal treatment. I won't need to stuff traps wall-to-wall to fight off those pesky nodes and nulls!
PS...love your avatar.
"We're gonna need a bigger boat."
Hooper!