As clear as a blue sky.
To add for this, something bugs me about the Beastman Country attacking Dragon Kingdom. It was mentioned that the Beastmen tribes were not interacting with each other before and only occasionally raid DK for food (delicious humans). But now, it seems they're well-organized and aims to conquer whole DK. My theory is that someone (most proably a Player) united the tribes and is leading the country to war with DK. I'm really looking forward for other Players appearing in NW, as Maruyama stated that he originally planned to end LN in around 18 volumes but has enough idea and/or material to go for at least 50.
Be informed that when the author covered NW power level, he only mentioned humanoids so the statement "Overlord of the New World" only applies to HUMANOIDS. Within the NW power level list no dragons were even mentioned of which would be categorized as HETEROMORPHIC. My guess is that those "True Dragon Lords" who did not join the war against 8GKs might pose the biggest risk as they might be still around somewhere in NW. DDDL is one of them, which managed to slay a Player equipped with one of the "Twenty" WCIs. Maruyama even stated that somewhere in the latter parts of the LN a great war would ensue which will involve countries outside the continent (current story only covers relatively smaller area of NW and numerous demi-human countries far beyond the Beastmen Country still exists out there) against Nazarick.