I don't think there is enough market for a ground up 11 or 12 abreast plane.
Designing one in cooperation sounds like a recepy for miscommunication, errors and budget and planning overruns, so very unlikely.
I think a small stretch to 80m for the 777 is all that is feasible in our lifetime.
I think Airbus learned their lessons in the A380 wiring debacle and are a much more rounded organisation now than they were. Bringing the C-Series on property will only have widened its experience here.
At the end of the day Airbus and Boeing are more or less the same - they are vast technology multinationals that spend huge sums on aeronautical research. Both employ massive r&d teams and are engineering focused. Both are almost paperless and use very similar design software. Both make their money on eking out efficiency savings in the build process and both are profitable. Both are proven to be able to take an aircraft family and improve/develop/modernise it as per market requirements. Both have dominant market shares in different segments and both have seen major programmes adversely affected by needing to compete with the other guy when in reality the smart money was to cede certain segments and focus on others rather than having a product that competes in every class/segment. Both have been accused of acting with hubris and arrogance in the past, perhaps trying to prove something, whereas both are now just about the money and the value. If something doesn't have a value proposition, it wont get built. Airbus is a radically different, more diverse, more modern and less political animal than when they built the A380. In effect it became a lot more like Boeing is now.
You look at the way Boeing and Airbus now use a LOT of subcontracting for parts and components - this would have been unthinkable 30 years ago but its how major programmes are done now. So if Boeing can work with Shorts, Spirit, GE, Zodiac, Mitsubishi, Embraer, Honeywell, Leonardo and Northrop-Grumman why cant they work with Airbus?
Yes there are ethos differences and of course the language issue. There would be major challenges on how to divide work, divide rights to research and of cousre how to harmonise processes and software, internal resource management and supply chain management.
Such a thing would be utterly unworkable and completely unrealistic 20 or even 10 years ago but times change, and so has the market.
Lets not forget the Europeans have been working in consortiums to build massive aviation projects for decades - it isnt anything new. Concorde, Tornado, Typhoon and now the two major 5th gen fighter programmes are examples of this - littered with hubris and difficulties in the past but as i say, times change and nationalistic pride and predjudice isnt what it was.
Picture this -
Emirates come to Boeing and Airbus and get everyone sitting round a table. The project divides up as, simplistically, as Boeing designs the fuselage which is completed in the same way the 787 barrels are. Airbus is contracted the wings which are done how Airbus does them now. Engines from GP Alliance and RR. Emirates' parent group (IE: Dubai govt) invest several billion into the development costs in exchange for preferential pricing when the new plane is ready and takes the lead on providing MX services, directly or by franchise, globally to all users. Hell, you could even assemble it in Dubai if you wanted to. Would cut costs for sure as long as the Boeing/Airbus unions were prepared to accept it.