Historical fiction is a broad and eclectic genre. You don't have to write a story that is a copycat of what has already been written. So my fourth tip is to think laterally and customize tropes and conventions from other genres as freely and cheekily as you like. Remember that there are already numerous sub-genres that have done exactly that: romantic historical fiction, historical thrillers and alternative histories, which are part historical fiction, part fantasy.
Examples include Maeve Haran's The Lady and the Poet (historical romance) which tells the story of the relationship between John Donne and Anne More; Dissolution, the first book the Shardlake series of Tudor crime thrillers by C.J. Sansom and Ruled Britannia by Harry Turtledove, which is set in an alternate sixteenth century England which has been invaded by the Spanish Armada. Some books span more than one sub-genre - for example, Fatherland by Robert Harris is both an alternate history (imagining that Germany invaded England at the end of World War II) and a thriller.
This is good news if you are fan of a particular genre of writing, and can help shape and focus your ideas .Plot can be an issue for many new or inexperienced writers, and both romances and thrillers operate within certain constraints and conventions, which both limit your options and clarify your narrative goals. (A romance should be a love story in which your protagonist has to overcome a series of obstacles to be with their lover; a thriller should revolve around a quest or 'chase' story, with the protagonist seeking to resolve a mystery or use their ingenuity to avert disaster or achieve their goal.)
The literary historical novel is also having a renaissance, following the success of authors like Rose Tremain, Sarah Waters and Hilary Mantel. The bets are off here - you can experiment and delve into much darker or stranger terrain if you want to write in this form. The recently announced long list for the Baileys Women's Prize for Fiction included a number of historical novels. (The books in question are: The Strangler Vine by Miranda Carter; The Luminaries by Eleanor Catton; Burial Rites by Hannah Kent and The Undertaking by Audrey Magee. There are also historical elements in two other books on the list: The Flamethrowers by Rachel Kushner moves backwards and forwards in time, and The Lowland, by Jhumpa Lahiri covers several decades of Indian and American life.
So - the good news is that this genre can be bent and twisted and adapted to any form that you like. You can use certain conventions, you can make lateral connections, and you can subvert the whole lot if you want to.