Once you have completed the main story on any difficulty, you will be able to start over with New Game+, keeping previously gathered experience and weapons.
To begin your adventure anew, head over to the main menu and select New Game+. This will not overwrite your manual save files from your previous adventure. Note, however, that autosaves from your previous adventure will eventually be overwritten. We strongly suggest creating a manual save of your current game before hitting that New Game+ button, or your progress so far will be lost.
If you have completed the game but do not see an option to start a New Game+, try loading your current game first. Once loaded, do a manual save and head back to the main menu where you should find the New Game+ option.
You will be able to choose between Alexios and Kassandra each time you start a new run.
Starting a New Game+ will also automatically increase the game difficulty by one level unless you completed the previous campaign on Nightmare difficulty.
Enemy scaling will be enabled by default in New Game+ - if you do not wish for enemies to scale up to your level, you can disable this feature in the Gameplay options menu.
All character progression will carry over to New Game+, including:
Some elements will automatically reset when starting a New Game+ save, including:
Maps and discovered locations
Spear of Leonidas
You can restart New Game+ as many times as you want, as long as you have finished the campaign in your previous play through.