I feel like breakthrough would be improved a lot if A) we actually made the sturm offizer perform like an actual unit. I.e. rework it's stats, cost, timing etc, gave it veterancy since it doesn't have it, and lastly looked at the abilities a bit. B) Make breakthrough artillery not drop on the sector point, but somewhere in the sector and ONLY in the sector where you did select.
Yes, Breakthrough has potential but currently the Sturm Offizier is practically useless and Assault Artillery is broken (as half the shells land outside the targeted sector), making the doctrine pretty lacking except for Panzerfusiliers and the Jagdtiger. Sadly Elite Armor and Overwatch were even more in need of a revamp so we won't be able to see changes to Breakthrough anytime soon.
Shuffling OKW heavy callins around has been a discussion for quite some time now because most of them would thematically fit much better in other doctrines.
I think it would be a good start to give Overwatch either the Sturmtiger (moved from Elite Armor and replaced there by a Tiger I Ausf. E) or - less radically - the Jagdtiger (alongside the one in Breakthrough). Either unit would fit the doctrine really well both thematically and mechanically and adding a call-in as the 5th slot instead of an ability would avoid a munitions strain.
It would also potentially open up options to tweak the Jagdtiger a bit.