Are you aiming for exclusively historical flavour/added flavour or are you also adding in additional paths to specific events, alternate possibilities to leadership, etc?
I don't know how to change leaders or the politicians as yet and most events are purely historical. However there are/will be many events which you will have to decide upon. Some decisions are actually 2 bad choices and some will be ahistorical. Once I've gained the experience I'll look at a whole host of ahistorical changes maybe even turning on Britain herself! This will be some months away though.