It would need a structure though, and as a starting point for discussion, I'd suggest .....
Collectibles - Royalty -
- British - Monarchs
- British - Princes & Princesses
- British - Minor Royals
- British - Royal Estates
- British - Royalty linked items
- Europe - Monarchs
- Europe - Other
- Non-European - Monarchs
- Non-European - Other
Feel free to add to it, but remember there are probably already categories in books, ceramics etc, which are used for royalty.
Gaz