Christmas 1943, Ike has a heart attack and is replaced by Jacob Devers as SACEUR?
Marshall would have been more likely.
Marshall had pushed strongly for himself to be given command of Overlord.
If Eisenhower died in December of ’43 the time frame would have been extremely tight, Marshall may have been seen as General with the most knowledge and experience at that level of Strategy and Logistical organisation, not to mention inter-service and inter-allied politics.