They need to be careful that a twin aisle aircraft doesn't encroach on the 787-8. They need s 200-250 spread aircraft that in large version will give the economics of the A321Neo and the range and field performance of the 757-200. The smaller version needs to cover everything the 737-Max8/9 does at far lower cost.
Bit like the EMB170/190 but scaled up in terms of family commonality. The smaller one would be the big seller, but the larger one would allow 757 retirements and benefit from cost sharing with the smaller.