This is just fantasy not real. Add two tubes to the Steinway tunnel and add two tracks up to Queens Plaza. Then where the Flushing becomes three tracks, add one track (perhaps above the three tracks).
Now here is the weird part... Have the two local tracks as division A tracks and have the two express tracks as division B tracks. There would have to be a way to adjust clearance between passing trains if the new track (among the three) was converted to B division.
There would be only a few (three?) stations affected by the third track conversion. The express stops. Perhaps 74th st should be converted to express too. It would be weird having B-division express service. With A- division local service. In fact, the B division trains can even come up from Broadway through Queen Plaza so there would not have to be alterations west of the bisection.
Anyway don't be too hard on me, it is only a fantasy, but what do you think about this? Let's not be overly concerned about money in my fantasy.