FURTHER OPTIONAL RULES:
To make SF a little less of a game-breaker, each player may us Strategic Focus once per year -- either during the Spring or Autumn turns, not both -- due to the level of planning required to gain such an advantage. Of course, one could, for example, use SF during Autumn 1902 and again during Spring 1903, as the Winter allows time to plan (Summer is just too hectic to re-organize SF).
To make SF more of a game-breaker, a player may order SF to hold an otherwise empty territory which they own; this represents last-line defenses, civilian militias, et cetera, organized expressly to repel an invasion against them.
Also consider using SF to convoy units where no real Fleet exists -- consider it to take the form of co-opted merchant marine or fishing vessels, zeppelins, extremely tricky high-speed fleet maneuvers, whatever.
What are your thoughts on this?