The historic summary of the line given in "St Pancras to St Albans" (Goslin & Connor, Middleton Press, 2002) says that it was built as a connection to the North and South Western Joint Railway at Acton Wells. Another source I have says that the latter railway was constructed as a link between the London and South-Western and the London and North-Western Railways between Willesden and Old Kew junction.
Goslin and Connor go on to say that the Acton Branch was designed by the Midland as a way of connecting to the South of England. In return the L&SWR had running rights to Brent sidings.
So it seems there were no plans for any other branches.