Cover should show a very handsome version of a Sherlock Homes (victorian gentleman) and more prominently a beautiful Lady in a victorian dress with brown hair (Irene Adler), in a victorian London setup. There should not be an obvious romantic relationship between the two (e.g. no kissing pose).
Other features could be either a diary or a horse-carriage (as both play aprominent role in the story). As the book plays with aspects of a classical romance sory (e.g. "Bridgerton") and the Sherlock Holmes stories, the cover should reflect this to some extent to manage reader´s expectations. It should therefore not be too dark (untypical for romance) but can still be mysterious (as murders do happen and there is a crime mystery to be solved).