I'm looking for something like Overlord, where the MC is already powerful, without wasting time on growing powerful, he/she is on a quest to conquer world
Quite a few of fics on my list are kingdom building https://www.fanfiction.net/u/9071421/ Also you might wanna read Gems of Infinite Potential even though it's dropped.
Legend of the Galactic Heroes - the inspiration of Code Geass, a critically acclaimed novel with the finest characters around, few pesky antagonist and majority of its cast on different countries are fighting to each other, sadly there's no translation of it in this site(you have to buy it) though it has an anime(new and old) and manga adaptation(childhood).