I like the idea of opening the wall between the two living areas. Please know that I'm an organizer, not a designer...so feel free to axe what's not helpful in my comments here. :)

The fireplace occupies the most useful corner of the merged room -- perhaps the entertainment/media area would be on the wall adjacent to the formal dining room?

As an organizer, my first thoughts are always efficient flow (walkway and usability) and also built-in or planned storage. Is yours a household with a dvd or music collection? Or are you more online? What activities do you enjoy in your living room -- down time with the cute dog? entertaining friends? low key family time? Be sure that you plan for nice shelving or a bar/counter space for games or snacks or books -- whatever it is that makes your living room HOME to you!

Eager to see your "after" pictures, especially to see what flooring you pick. Have fun!

Yes...I was wondering about the wall shared between the merged living room and the dining room...? I don't know if you were also planning to open up that room or retain that structure for some usable wall space.

Natural lighting is great -- but the windows do limit usage of the walls. So many tricky considerations to this project!

It looks like you have some great spaces. In this case, I would bring in a designer that you trust to walk the space with you and look at options.