Wow - that's going to be so nice! Configuration depends so much on what you need to store (lots of shoes? longer hanging items such as dresses and coats?) and how you like to store your clothes (drawers? folded on shelves? on hangers?). But in general, it makes sense to me that you would do hanging rods (double would be my preference) along the 2 long walls, with shelves above the rods. You could put a 30" wide bookcase or shelving system directly across from the door - these are great for storing shoes in clear boxes, or folded items. Over by the window, on the 5' wall, I'd think about another shallow shelf system. It would be fairly tight between the hanging clothes on the long wall and the shelves on the 5' wall, but if you do really shallow shelves - like Ikea's Billy at 11" deep - you could get lots of good storage. An alternative would be to use the 5' wall with some sort of rack system, like a giant pegboard or slat wall. Then you could hang accessories, jewelry, hats, purses, or even an ironing board.

Have you looked at the Container Store website? Their Elfa shelving systems are on sale until early February. This would be a great time to design your closet and buy while you can save 30%.

Neat. That's large enough to be considered a dressing room, I think. Since I struggle with a 42" x 14" closet, I'm really happy for you to get that space.

I think the first thing you need to do is look at the clothes that you have. How many feet of hanging space will you need? Double rods will save a lot of space, but that depends on how you hang trousers and such--by the hem with those clamp hangers, or folded over a hanger? How much long hanging space will you need for long dresses and skirts and coats? Given that you will have more hanging space than you did before, what do you have folded in drawers now, that you will be hanging in the new closet?

You might do a consultation or two with a closet company or the closet designer at a home store. They will have the knowledge of specialized storage units that would work well for you.

My first instinct is to do double rods on the 12' wall, a single rod on the 5' wall, with shelves above everything. (I'd put all the out of season clothing at the window end of the closet.) Then go wild with shelves and drawers on the 8' wall. Put a full length mirror on the wall that the door opens against, and a bench or stool somewhere to sit down and put on shoes.

I've added a link to a video of a woman giving a tour of her closet--the configuration is a little like your space, except that you might have more room. It's long and she rambles a bit, but she does a decent job of explaining what's stored where and why. There are many other closet tours on YouTube that could give you some ideas.

Here is a link that might be useful: organized closet

I would configure it this way...
TO RIGHT OF ENTRY: small but sturdy stool/bench; to right of door
LEFT SIDE: 16" deep shelves with dividers - store folded sweaters, etc. and/or use linen bins and store items (camisoles, purses, pajamas) inside the bins. At 5' space, small hooks for hanging necklaces, and other jewelry, scarves, etc.
RIGHT SIDE double poles with 1 shelf (80"H)with a small portion of that side to be a single pole.
WINDOW WALL: If there's room, shelving above and/or below.