No way, José. Absolutely no room to squeeze another pier in between Pier A and the Eastern Arm. You would have to lose berth ED2 on the eastern face of Pier A and berth ED1 at the landward end of the Eastern Arm so it would be self-defeating. Two of the four piers (B and D) in the top left of the photo below are redundant as it is, legacies of a day when ferries were a fraction of the size of today.
You need a fair bit of searoom to berth a ferry safely without damage in the immense and varying forces experienced in high winds, with the strong circulatory currents in the harbour another factor. It is not like reversing into a parking bay at Tesco.
There is no requirement for further ferry berthing capacity in the Eastern Docks. The limiting factor is the area of water off the ends of the piers. Only one vessel at a time can swing to go astern into her berth. This swinging area is fully utilised and queues of ferries waiting to enter and depart build up at busy times as it is.
This is the whole raison d'etre for the plans to build Terminal 2 in the Western Docks. Nothing to do with the ability to handle extra traffic landside in the Eastern Docks, everything to do with the amount of water available to manoeuvre the ferries in.
In the photo below, one of the giant Maersk (now DFDS) ferries is seen swinging off the end of Pier A to go astern to berth ED3. The bunkering tankers Clipper Bordeaux and Coralwater are seen on the South Jetty (Dover Cargo Terminal) and Eastern Arm respectively.
The photo below shows the view from the Eastern Arm looking across the linkspan for ED1 towards Coralwater being topped up by Clipper Bordeaux on the South Jetty, with Pride of Calais in ED2. The modern temperature-controlled storage sheds on the DCT can be seen, together with one of the new mobile cranes, and one can only devoutly hope that Hammonds are able to find new customers for these facilities.