>>5014647
I've imagined a much larger descendant of coconut crabs inhabits an island without much in the way of predators, where they could evolve to be much much bigger thanks to their specialized lungs.

Although they might need some help in the way of evolving legs that could support such a hefty arthropod on land. I also imagined these crabs having seasonality, since the drier parts of the year could be hostile to them or just unable to find necessary food.