You can't have an RPG-like without something huge to bring down. So why not have a bunch? You could have either a chance of a huge named behemoth of a creature spawning nearby, or boss arenas generated during planet generation, with said huge boss creatures to fight within. They could be either a giant version of an existing creature on the planet (think Skagzilla from Borderlands), or a uniquely generated monster with no apparent origin (think King Ghidorah of Godzilla fame). To make it more interesting, the creatures could also be generated with non-organic parts, like an automatic flak-cannon that was ripped off a starship. The bosses could be air, land or sea-based, and could even alter the environment i.e leaving a permanent tunnel in the surface, draining away water from an ocean. Infinite giant monsters would be amazing in a near infinite universe.
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http://community.playstarbound.com/index.php?threads/massive-bosses.24706/ I think this suggestion is pretty similar. I suggest looking it over and seeing what concepts match, and then trying to make yours stand out.