It is exciting and stressful, but all in all fun!
As for the theater, I do have it all in my head at the moment. On my first theater, it was the same way.
Just like you said, it will change and change again and again before I even start.
At the moment I want to put a coffered type ceiling with soffits, crown molding and some type of accent lights.
I will most likely not put any poly coating on the wood, I would not want any reflections. I have none in my old theater and even 11 years later, the oak looks good with just a stain on in and no clear coat.
I will go with a darker carpet(grey or the like). The walls I would also like the nice wood column look, but in between I will put some type of fabric up. I will go with the 7.2.4 Atmos type set up.
As for were my electronics will go, I will probably put them in that little bump out part. I would like to have them in a separate room all together, but the runs would be pretty long. So I might build it into the wall and then have some type of paneling or fabric door covering it up while watching a movie. I have not figured that out yet.
I will go for sure with blacks, greys and maybe some deeper reds.
I will also sounds proof the room more than my first theater. Might go with clips/double drywall/GG/etc. 60% of the walls have concrete behind, so not sure how much sounds proofing you need for them. I would assume not as much, but I might be wrong.
The 2 row of seats will be 18-24" higher than the first row, since I did 10" last time and that is just not near enough and having 11' ceilings this time, give me a lot more options.
I will put a 2ft false wall in the front to put my speakers behind, I am leaning toward a Falcon screen and he said he could custom build one in the 180" range(2.35/2.40:1) and then have side masking.
As for the projector, I like the Sims2 Sirio at the moment, but I know that will change, since I wont be buying it until about this time next year.
I know a lot of people on this site will say "do not go to big", 40 Deg viewing angle etc, but after 11 years with my theater and having a 150" 2.35:1 screen at 15ft back, a 180-190" 16' back(where my head will be ) with the 11' walls and 3 extra feet wide, will fit really well(for me anyway haha)