I just thought of a situation ,
As each ENB does S1 connection with MME.so consider 3 ENBs(those are nearer to each other) did S1-Setup with a particular MME.
After S1-Set up done can we have/add some intelligence (Based on some identity ) at MME to know who all are neighbor for whom?
If we will know about this we can make X2 -relation with neighbor easily ,so that we can reduce the overhead of getting info of neighbor ENBs.