Some little ideas.
I'd really love to see actual constellations on the night sky (with the possibility to get maps about it with special quests). Some could represent mighty creatures (bahamuth, leviathan, gilgamesh, etc.), others should represent the Twelve.
And depend on the area you are, you see different ones in the sky. Devs could put some benefits when you are under a constellation that shows your deity (you choose in the character creation). When the weather is cloudy, the benefits could decrease a bit.

For the moon, i think it has already an actual influence with its phases but dunno which sincerely (for crafting?). It should have influence on fishing for sure. Maybe some seaside areas could be affected by the moon, with low and high tide (some paths could be open or close). Some normal monters or NM could appear with full moon.. and so on.