Boeing has a huge problem, and it is called 321NEO. No matter what they do from scratch, it is going to cost a lot more than a 321, as Airbus has fully amortized the cost of development a long time ago.
They will have to knock it out of the park with efficiency to be able to sell many. Probably at least 20% better than the 321.
I always thought they screwed the pooch with the NG. When they redesigned the wing, they should have put longer landing gear on it. It would still be a guppy, but at least it would still be relevant in sales. The NEO is killing the MAX, and it will kill any new design on cost.