Room acoustics in the construction phase is a pretty deep subject. The sky is the limit for how far you can take it.
As for the room shape, that really only impacts low frequencies. There are things you can do to mitigate that. I wouldn't be worried about room shape so much.
You definitely do not need to add cement shapes into the room.
If this is a plan that you intend to invest a lot of time, money, and effort, you might just want to hire an acoustician. There is a lot to learn.
A good place to learn about acoustic is in the book
Sound Reproduction by Floyd Toole. He has a great supplementary site that is some very good reading for what you want to do,
here and
here. Definitely read his papers about designing a home theater or listening room.