And I gotta admit, container/cargo capacity is the second biggest question mark in my mind. I don't know how Boeing would manage to fit in standard cargo containers. A custom design might be required and wouldn't be unprecedented. One thing is that I don't see 797s flying routes with a lot of bulk cargo demand - rather Hub to Hub or TATL from EWR/IAD/ORD to smaller European cities such as EDI. So, less cargo capacity than a 787/777 might not be a major issue.

Biggest question in my mind is engine placement. A lot of advanced concepts these days have them above-wing near the trailing edge, as there are quite significant weight and balance benefits. While we might like looking at the engines under the wing ourselves on the walkaround right now, I think future engine generations will be able to detect and automatically report problems well before we ever could spot them. And hopefully they won't need oil top offs on a regular basis either...