A lot of people don't realise on their first playthrough that a lot of the bosses are optional, or that there are likely a lot of them that were likely missed. It's pretty inevitable due to the way the complete lack of handholding and relatively little signposting the series has, and in fact, it's one of the best reasons to complete a NG+ run; to take on the optional bosses missed. Dark Souls 2 opened us up a bit more to this with Belfry Luna, but sadly failed to expand on it. Secret and optional areas have been a mainstay of RPGs, and it seems odd that Dark Souls would benefit from this more. Secret areas with secret bosses, multiple routes that offer different sorts of aesthetics and enemies, or even some more expanded side quests, would be a great way to add more depth and character to the world.
Currently living the dream by studying in Wales and writing articles about the things I love for beer money. My proudest achievements are teaching myself Accordion and getting my head round the off-side rule.