If it was me I would do the bedroom in the back but I would tuck a small walk in closet behind the bathroom so that the bathroom wasn't so long. 15 feet seems rather long. I would do what you suggest with the fireplace and shelves between 2 flex rooms and leave the front of those rooms open to the middle area so it feels open and spacious. I assume that you are closing off the mechanical room where the furnace and hot water tank are. That corner might be a good place to put a wet bar. Hope this helps

In most places, check code, you will need a closet in the room for it to be considered a bedroom and not a den. Since the bathroom is long, you can reduce it to 10' and have 5' closet next to it. Also it looks like there are a furnace and water heater that woud will want to enclose, maybe you can add another close next to it, instead of the long closet outside of the bedroom. As far as the rest of the space, it really depends on what you are planning to do with it. What is the general idea of the use? will you watch to TV or host guests? need more storage? answering those questions will help you select the correct layout for your needs.

The need for a closet in a bedroom is an old wife's tale started by realtors for listing in the MLS service. There are no such "codes". The International Residential Code requires a sleeping area to have an emergency escape and rescue opening, smoke detector, CO detector outside the room, a switched light, but no door,... let alone a door to separate clothes from the rest of the room. The "CODES" do not even use the word "bedroom", let alone require one to have a closet.

The "long closet outside of the bedroom" is actually INSIDE the bedroom if you look close enough at the pencil drawing (picture 2) instead of the beam on the print.