Airbus' weakest program is the A330neo. In addition most are wagering that Boeing's next project will be the MoM, and Airbus already has some market penetration in the lower part of that market segment (with the A321LR/XLR).
So I don't think Airbus will do a MoM (that risks direct competition with Boeing and cannibalization of some of their own product).
I think instead they'll replace the A330neo. It'll probably also be a good Dreamliner replacement/competitor and have 8-abreast economy seating at A220 dimensions, and 7-abreast premium economy seating.
Maybe a wide seats, spacey 1.5 aisle fat narrowbody could neutrailze typical narrow body disadvantages, while benefitting from it's efficiency in the 250+ medium range segment..
You would need 40-50k lbs engines though.
Your A370 would be more than 16% heavier than the A321XLR per passenger, even in the most favourable configuration. Wing loading will be better but I think you simply over-designed the aircraft. Just put a new wing which is about 20% larger wing on the A321xlr and a longer landing-gear and create a new simple stretched version, that will be enough. It will cost halve the development costs.
Interestingly, the wing on the A345/A346 is 20% larger in area than the wing on the A342/A343. It incorporated a three-frame (1.6m) centre wingbox* extension combined with a tapered outer wingbox insert, along with 1.6m wingtip extensions and the 2m-span canted winglets from the A333/A343, and increased fuel capacity by 38%. The tapered insert provided a slight increase in wing sweep, from 30° to 31.5°, and greater chord without any significant increase in depth -- yielding a slight increase in cruise speed; from Mach 0.82 to Mach 0.83.
* The A333/A343 centre wingbox is 5.33 metres long (i.e. equivalent to 10 frames).
* The A345/A346 centre wingbox is 6.93 metres long (i.e. equivalent to 13 frames).
Now, if Airbus were to "copy" what they did with A345/A346 wing and use a similar tapered chord-wise wing insert on an enlarged A322/A323 wing, the development costs would be much lower than what is required for an all new wing.
The length of the A318/A319/A320/A321 centre wingbox is 3 metres (i.e. equivalent to 2 full frames, 2 half frames and 2 emergency door frames; or 5.66 frames ). Hence, a 2-frame (1.07m) centre wingbox extension combined with a tapered outer wingbox insert, along with slightly modified and wider 1.61m long neo
wingtip extensions and sharklets, would increase wing area by 19%, which combined with the larger centre section, would boost fuel capacity by up to 40%; or by nearly 10,000 litres.
The enlarged wing on the A322/A323 would use the the same trailing edge as the A321XLR. Hence, the enlarged wing on the A322/A323 would have an area of about 153 m2 -- vs. 124 m2 on the A318/A319/A320 and 128 m2 on the A321.
Now, an A322/A323 enlarged wing would have a significantly lower aspect ratio than the current wing -- that is, if the existing neo
wingtip extensions and sharklets would be used instead of new folding wingtips:
In fact, if the enlarged wing of the A322/A323 would be outfitted with 2 x 3 metre long folding wingtips*, the in-flight wingspan would increase to 41.8 metres (35.8 metres when parked and taxiing) and the wing area would be increased to around 160 m2. The wing aspect ratio would then be nearly as great as that of the A330-900.