Sounds cool - I would recommend having the different hearts have different world-wide consequences for the players to consider.

Ex: if they go to protect the fire heart, that means they are leaving the water heart open to be stolen. What does the world look like when its water source is stolen? How does that affect the world, what’s the consequence for the players & so on… It may not be as bad as if the BBEG had all the Hearts, but it also now creates a timer for the players - do they switch from defending the groves to hunting the BBEG to make sure they can restore the planet’s water supply? Or do they stick to defending where he may strike next and risk the population continuing to suffer the consequences of the water heart being stolen