I guess the question comes down to ..How to compete with the B777NGs and what to do about the gap between 380 pax (A346) and 550 pax (A388) or more precisely the larger gap between 300 pax (A359) and 550 pax (A388).
Possible solutions would be
1. Come up with a new cross section
2. Close the gap by lengthening the A350 cross section and shorten the A380 cross section
I feel airbus would go for the second option.
Possible to observe two clearly different policies for Airbus and Boeing
Tends to focus more on what is best for the present and it seems to pay them off quite well.
For example B757, B767, B777 all are quite successful and B787 will also be OK
(I guess so)
Also they tend to disregard consistency.
For example see the number of different cross sections.
One problem with this is they have to maintain separate lines for each cross section and when sales go down they will have to close down the line
Seems to focus more in the long run regarding the optimum cross sections, and then try to stick onto them.
For example the 8 abreast and 2 LD3 cross section they came up with A300 in early 1970s seems to be one of the best in the class. The fact that B787 has chosen it proves the point.
Cross section they selected for A380 also seems to be quite efficient
Selecting the right cross section and sticking to it seems to be a significant advantage to them as they can produce multiple models with the same line and they need not close the lines even if the sales are poor
Considering these I feel airbus is more likely to have only three cross sections for the time. ( the A320s, A350s and A380s)
So the decisions would be
1. Shorten the A380
It will be too heavy and will not be economical if the present wing is used.
But if a new wing is developed, they might be able to overcome this problem.
In fact they can develop a wing optimized for two different A380 lengths less than 73 m
Developing a whole new wing is costly but in the past they have done such things.
For example groups of A300/A310 and A332/A333/A342/A343 and A345/A346 and A358/A359 have different wings
So why not one wing for A388/A389 and another for shorter A380s
They tend to keep the same cross section but adopt quite a few wings for different versions
With these shorter A380s they might be able to compete with B748s.
I feel they are waiting till Boeing is stuck with the B748 (design freeze or what ever)
2. Improve the A345/A346 with A350 technology and when suitable engines are available they might come up with two twins having lengths of about 70m and 75m to replace the modified A345/A346.
Or they may produce all. That is quads and twins with similar lengths because they can use the same production facilities due to common cross section.
If I remember correctly, they still can produce A300s
So with all these reasons, I feel they might go for something like
A380-900 (656 pax) (79.4 m length) (79.8 m wing span)
A380-800 (555 pax) (73.0 m length) (79.8 m wing span)
A380-600 (510 pax) (70.0 m length) (different wing may be 70m)
A380-500 (450 pax) (66.0 m length) (same wing as A386)
A340-900 (387 pax) (75.3 m length) (63.4 m wing span) modified A346 with new materials and incorporating other features in A350
A340-800 (320 pax) (67.9 m length) (63.4 m wing span) modified A345 with new materials and incorporating other features in A350
Above two quads might become twins when suitable engines are available
A350-900 (300 pax) (65.2 m length) (61.1 m wing span)
A350-800 (253 pax) (58.8 m length) (61.1 m wing span)
A350-600 (222 pax) (54.0 m length) (different wing may be 50m)
A350-500 (185 pax) (50.0 m length) (same wing as A356)
All these are just what I feel, So I could be wrong.