Here are my current ideas about the regalia. One thing I didn't maybe make clear in my initial description of the items: they belong to the Sorceress, not to the king himself. They're the regalia of the King's Sorceress, his sister and mother to the next king.
The reason for the two items that require a dragon - the royal family at one time did have a dragon ally. They lost this ally (a silver dragon) some years before their kingdom was overthrown, basically because of their own pride. This was about 400 years ago, campaign time, so the dragon, or its heir, might still be around somewhere...
Regalia
Ring:
Non-sorcerer: has no magical powers, acts as a ring of Nystul’s magic aura
Sorcerer: +1 ring of protection
Sorcerer related to royal family of Tremon Arivia: +2 ring of protection
Wearer can cast 5th lvl spells: gives +2 chr bonus, but must first be placed on the hand of the wearer by the true king of Tremon Arivia
Wearer can cast 9th level spells: grants user 3 wishes, but must first be immersed in the blood of a still-living dragon.
Scepter:
Non-sorcerer: can cast detect thoughts once per week
Sorcerer: can cast detect thoughts once per day
Sorcerer related: can cast suggestion once per day, and remove curse once per week
Owner can cast L5 spells: can cast lesser geas or sending once per week but first the scepter must be blessed in the High Temple of Khurabai, patron of the kingdom of Tremon Arivia (a god from my own campaign).
Owner can cast L9 spells: Can cast Demand once per week but first the gem in the head of the scepter must be held in the mouth of a living dragon.
Tiara:
Non-sorcerer: wearer of tiara appears always to be freshly and neatly dressed, groomed and clean (but actual style, quality and mode of dress are unchanged).
Sorcerer: as hat of disguise
Related Sorcerer: as hat of disguise and allows invisibility once per day
Wearer can cast L5 spells: as ring of invisibility, but must first be placed upon the head of the wearer by the previous high sorceress of Tremon Arivia.
Wearer can cast L9 spells: the tiara allows the owner to dimension door 3x day as the spell, but first it must be placed on the heartstone of the kingdom of Tremon Arivia for 24 hours.
What I am planning for currently is that the players will soon find the ring, and the journal of the Princess Valencia, who was daughter to the last Sorceress of Tremon Arivia. The princess and her brother escaped the falling kingdom, and she hid the other two items of regalia, but kept the ring with her, until she died in the place where the PCs find the ring. She mentions in her journal that there are multiple powers, but not what they are or how to invoke them. This should create a fun and ongoing quest for the party.
Later they may have to search for the King's regalia, or possibly recreate them!
Thanks.
Gilladian