I know many have asked for this and the only problem that DF could possibly have with this is that the premium bonus created by a 7 man squad would be too OP.
So I propose giving us the ability to add to and customize a premium squad. However only the original premium soldiers will be used for calculating premium bonus.
So this will effectively not make the squad OP with regard to premium bonuses and any addition soldiers added to the squad will generate normal XP.
begining at lvl 20 in the campaign let us increase the premium squad size from 4 to 7 soldiers. In addition gives us a tree where we can upgrade as a normal squad
Difference is, whether they would want to prevent it from happening or not.
The “No” (to restoring their composition) as an answer is obvious to most of us, players, but I have my doubts that devs want to see it same way.