Jan 10
Animal and Plant Exclusivity
In real life different continents have different flora and fauna, could be fun to have some plants or animals to only spawn on one landmass, even if it's a small island or something. With more animal and plant variety this could encourage more trade, interaction, or exploration for smaller servers.
Pending
Not quite what you're suggesting: but I think it'd be neat to have one near-mythical animal species that has ~8 members of its species in the world, total. They could hide from players impossibly well (they perfectly detect players through walls and start running from as far as 25 meters away). They'd require a coordinated effort from multiple players cornering to hunt and kill, but give valuable hunting trophies when harvested, i.e: big Living Room XP rewards. Instead of multiple copies of the same trophy giving diminishing returns in XP, having copies gives MORE than 100% XP per copy (for a species with 10 members, hunting 2 of them and mounting them both on your wall would be pretty prestigious after all).