A stronger independent Royal Marines in the seventeenth century

Could the Royal Navy have developed the Royal Marines into a larger, formally separate service in the seventeenth century?

Yes, but chiefly if government distrust of the Army created demand for an alternative armed force; otherwise Army opposition would constrain expansion.

A major political rupture between government and Army provides the proposed route to a larger, more independent Royal Marines. Government could fund the Marines as an alternative force when it distrusted the Army. Without that pressure, expansion would create a dispute over roles and resources. The US Marine Corps' separate position is interpreted as a later example of political balancing, although the practical effectiveness of that balance remains uncertain.