Boeing sent out an RFP for a 45K-50K lbs engine with less than 7000lbs dry weight, and is a minimum 25% more fuel efficient than the Pratt PW2000. That means the 797 would be smaller than 767, but slightly larger than a 757. If they simply re-engine the 767, it would be cheaper to keep it a 767, than certify a new airplane the same size as a 767. It's definitely feasible to build the 797, the problem is the engine manufactures will take 10 years to get that clean slate engine, that was requested in the RFP. Boeing could simply go with the current Pratt PW2000, but the A321XLR would destroy it in operating costs.