I was talking hypothetically. Instead of creating a new problem (Perambur terminal south of suburban lines) to solve the existing one, a better way would be just linking all suburban lines at Beach and making MAS a dedicated railhead for only long-distance trains.
Only disadvantage is It will reduce the number of goods/regular trains plying via MSB.
West-bound...
more... regular Trains from MAS could halt at Perambur, Avadi, Tiruvallur etc. while North-bound trains could halt at Tiruvottriyur etc for passengers in the suburbs.
With Avadi, you have a chance to make it a multi-modal hub if the State government can be roped in. Ambattur has accessibility issues which can be solved by creating an entrance from the North with the CTH road.