As someone who only discovered Zombicide Black Plague recently, I have just started to get hold of the add-ons that dramatically effect the difficulty of the game.
To date many threads exist with advise on how to build your deck out, to make is harder/easier.
I decided to build a tool that essentially allows anyone to select or remove any cards from the 160 available, in order to evaluate the frequency they might expect to see certain card types in a game as well as monster types at each danger level.
There is also a 'Predicted Difficulty' flag built in, this is very rough indicator based on what you have selected and feedback on threat weighting for each Zombie type will likely need plenty of refinement from experience players.
Let me know what you think, and a big thank you to 'Tarod' for compiling a spreadsheet of all Zombicide Black Plague cards created to date, this has been invaluable.