I would love a political sort of intrigue line set in a fantasy/psuedo historical place. Rebellion, the genocide of a complete family save for one child, arranged marriage, age gap, and the throne of a kingdom. I would want to be neck deep in this down to making family coat of arms and maps.
I have several variations on the theme in mind.
Variation 1 My character (from here on out known as MC) is the youngest child of a great noble house. The family rebels against the crown for some noble reason and it goes badly for them. MC was to be killed as well but the (adviser, brother, other such important person to the monarch) wishes mercy for the innocent child and takes MC into their home as their ward. MC grows up knowing only kindness from your character (from here on out known as YC). There is at least 15 years difference in age but something draws MC and YC together. Now an adult MC's want for revenge is tempered by the feelings of devotion and potentially even love for YC.
I would prefer for MC to be female but it could easily go the other way as well. YC is probably a widow/er or never married for some reason and their relationship since MC started coming of a marrying age has walked a very thin line of propriety.
Variation 2 MC is the youngest son of the king. The family follows the old religion, something that isn't quite trusted any longer. A rebellion begins and the royal family is murdered by the usurper. As the castle is taken the queen is in labor with MC and begs for the leader of the rebellion to let her son live. The leader tells the queen if the baby survives he will raise it as his own, not expecting the baby to live. The queen places a curse on the leader in an effort for him to keep his word. The baby survives but the leader doesn't keep his word and sends the boy off to be raised in seclusion and become a member of a celibate order so the family line dies with him.
The queen's curse enacts itself by keeping the newly crowned king from having a male heir. As a last ditch effort he sends for MC to impregnate his young wife. MC manages to be around for far longer than the king expected and intends to steal the throne by seducing the king's daughter (YC)