Some suggestions:
The Technocratic Union: create a flag for a nation that sucessfully becomes a Technocracy and a Major Power - it can be in any nation and with any definition of "technocracy".
Alternate national animal: change the national animal of a country and make a flag of such nation with the alternate animal on it - for example, a Germany with a Bear rather than an Eagle, or a Scotland with a Wyvern rather than a Lion.
The Filibuster Republic: the XIX century was full of ambitious adventurers, personal armies and unstable non-civilized regions. The result of these things was some very short-lived and unusual nations, such as the Republic of Sonora, the Republic of Acre and the Kingdom of Sarawak. Create a flag for a (OTL or ATL) "Filibuster" state that manages to survive - it doesn't have to survive until present day, but must exist for over 30 years.