Taking the idea in a different direction...
You commit 2 builds (at the same time, they cannot be spread over 2 build phases) to a Fleet with Marines. This fleet then strikes with a force of 2 when moving into a land zone- make the actual effect be that the Fleet stays in the sea zone, and the Marines then move into the land zone, with the Fleet's support. This is a one-time action, the Marines cannot then re-embark onto the Fleet, and acts in all respects after this turn as a normal Army. This opens up some interesting tactical options- a single defending Army is overpowered, but one support defeats the landing.
Some questions are raised. Would the Army be lost, KIA on the beaches, or would it remain on the Fleet to be used another time? What would the strength of the Fleets carrying Marines be: 1) on the open ocean, 2) supporting another beach landing?
My own opinion... a failed beach landing should result in the disbanding of the Army, the Fleets should remain as a single unit in strength on the open ocean, but supporting another beach landing they should count as 2, otherwise committing 2 builds to multiple Marine Fleets is a waste of resources.