I felt like the ending of Black Clover was a little rushed since a couple of concepts were not fully developed or explored. Not a bad ending overall though.
Ichika handing them the language converter for OTHER COUNTRIES indicates that there are definitely other countries/continents unexplored and untouched. I feel like there might be a spin-off series like Fairy Tails 100 Year Quest, where they are introduced to friendly and antagonistic civilizations. Now that the four kingdoms seem cordial with each other and have helped in big wars, there could be more foreign problems. I'm not certain if the series will definitely end here, but there is room for more world-building. Perhaps the flying aircraft at the end of Ch 392 was from a different country attacking?
Either way, it would still be cool to learn more about Diamond Kingdom, learn about what's going on with Spade ( we still don't know what Yuno's mom wanted to tell him), Dwarves, etc...