You should check out some of the previews that dropped last week. A few of them talk quite a bit about customisation and it seems like its quite extensive.
Off the top I recall reading you can customise Arthur's facial hair (grows in real time), theres a layering system with clothes and no such thing as outfits meaning you can mix and match shyt up, you can also choose wether to tuck your pants into boots, roll sleeves up etc. not to mention crafting clothes from animal skins. Theres a whole bunch of things you can do to weapons as well like engraving them, changing barrels and grips etc. Can also customise your horse's gear as well.