There IS a new fleet size constraint on extending the Hertford-Stevenage service through to Letchworth. It would require two extra units to maintain a 30 minute interval service, partly due to the lengthy turnback procedure at Letchworth via the yard reception line. If there was stock available for this I don't think the supposed capacity constraint is 'simply' trains per hour but rather that the trains from Hertford just happen to be arrive at Langley Junction at a particularly busy part of the half hourly cycle on the down slow line, making slotting the paths in difficult without an unreasonably long wait at the junction, which could in turn also result in passengers from Hertford missing onward connections for destinations beyond Hitchin and Letchworth. It's possible the Hertford trains could be retimed to avoid this but that no doubt would require a very large scale rewrite across all services and TOCs all the way back to Moorgate, Kings cross and the Thameslink core! Keeping things simpler for future Thameslink service reliability is a particular concern and clearly with the new service density there's no possibility whatsoever of continuing the practice of reversing the Hertfords on the down slow in platform #4, with reoccupation allowance in the region of 10 minutes for that manoeuvre. As an ex Letchworth resident, I'm disappointed a direct service, at least off peak, to the County Town can't be maintained, but good reliable connections at Stevenage are an acceptable and sensible compromise. The obvious position for the new independent line is on the down side from the junction, and there is already a left handed crossover just Hertford side of the underpass on the branch to allow a London bound train to cross to the up line (used by the platform #4 turnbacks today). A simple six car independent terminal platform could be tucked in at the south end of the station between the down slow and the retail/entertainment car park, clear of the existing footbridge. One thing I don't know is whether Six Hills Way overbridge (the first bridge south of the station) would need reconstruction to accommodate the additional track, but Broadhall Way, the next major structure, should be OK as there are 5 tracks beneath it already.
Edit: GB confirms there's room for the additional track beneath Six Hills Way bridge