I like the idea of tufting on the back. I would recommend a grey fabric, perhaps with texture or pattern. It looks to me, from the pictures that the rug in the room doesn't go well with the beautiful chairs and furniture you have in there. It could just be the picture though.

your living room's style and colors really will dictate fabric that you select for this sofa. without mire images and details, it is impossible to help you.

I think the tufting looks nice. I assume that you plan to use the same style of tufting.

If this is going in the same room with the tufted white sofa, I recommend NO tufting. Tufting can be overdone, and one major piece is usually enough.

The warm tones in your rug really work against the cool tones of the wall and upholstery. I'm not finding a fabric that looks good with both.

so Shall I stay with the original design and solid gray fabric? My corner fireplace wall is in Gray. Carpet is expensive and custom made. Hard to sell it for what I have paid. it has floral design on it.
what about the handles? Fabric on the other sofa is cream color and I still have about 15yrs. left. Thinking of using it but it doesn't have to be that fabric.

15yards.

HI -- I love what you have so far but .....I don't like the wall color with the chairs .. too weak . For the new piece I think you could either match to the other sofa and or that same color for the background of a small ticking with black stripes or do it in a richer teal or navy or charcoal, plain velvet micro fiber . I could also see color from the rug , possibly a burnt orangey/rust in a velvet micro fiber fabric .

I would do a two piece cushion back in an aubergine plain micro fiber With no skirt and round bubble type legs with steel casters on the feet.

I would not tuft the back.
I would consider the same fabric you have on the other white sofa. You have a matched set of chairs. This assumes the two sofas will be in that room together. Which seems important to me. Don't ask me why. I answer from my gut.

I would match the fabric to the couch you have. They will go with anything in the future if neutral. I would have matching pillows (throw pillows on teal on couches and cream on chairs. I would find a larger lamp in the perfect teal color. I would keep the walls neutral but add large painting with teal color over couch. For any body that tells you blue/green and brown don't go together should go outside. But I think you need to get a LOT more teal in the room to balance out that rug. Beautiful furniture!

change the rug.

The rug is the centerpiece of your room. It will go with anything. I would not go with another tufted piece. Let the one you already have be special. Check Houzz for sofas to get an inspiration for the style & I suggested going with the darker color from your beautiful rug for the fabric color.