I know it is far too early to speculate, but it's fun to imagine what ideas SE has in store for us in 4.x.
We have had Bahamut and Alexander, arguably two of the most powerful eidolons (espers, summons etc.) in final fantasy, so what can they do next?
I was thinking that perhaps the next raid could have something to do with omega weapon, and perhaps we would either ally with shinryu, or fight both of them in a 3 way war. The only issue with this is that with the current lore for omega weapon, this would likely be another Allagan raid, and I can see SE wanting to move away from that.
What does everyone else want (or think) the 4.x raid to be?