Frontier SMP is a Java Edition server for 1.21.8 and it is built around exploring a world that feels huge and wild.
The terrain is custom made so biomes feel bigger and more adventurous with massive mountains and big structures that stand out.
The main thing is the altar system. Altars are hidden inside large structures around the world and if you spot something massive you are supposed to search the whole place. Finding an altar can give you power like abilities and upgrades that change how you play.
It is a true survival style server that stays vanilla based but adds the altar mechanics. The vibe is competitive and strategic since exploring more can make you stronger.
