Clerics revere the spirits. They have one particular spirit or class of spirits they are especially close to, whether they are just aligned with them or they personally serve them. They are the intermediaries between mortals and the divine, and they typically serve people and seek to maintain some balance between the needs of people and the needs of the spirits. Their wisdom helps them to know what is needed to implore a spirit to aid them, and each spell cast is one such request.
A druids magic comes from the spirits, but where a cleric reveres one spirit, a druid respects them all. A druid's concern is more about the spirits and the balance of nature. Civilization is not specifically their concern, and often civilization is the enemy of nature. To a druid, the spirits are their allies and companions. Like a cleric, druidic magic is made up of requests of the spirits.
Thanks for sharing. It seems the cleric and the druid are already differentiated in your world, the cleric worships powerful spirits and interact with other mortals, the Druids protect the nature and the lower spirits they find, they also prefer isolation to civilization.
If what you wanted is suggestions I have one, add spice to the background. For example make the gods or high end spirits more involved in mortal affairs and consequently the clerics more radical in their beliefs, with sins, perhaps following orders of gods/spirits at war or being puppets of a mighty yihad, with concrete goals in the world. Perhaps the sun spirit wants to burn the world and destroy other spirits, each month heat increases, there are more natural catastrophes, the pope of the religion has started some holy wars persecuting minor religions as heretics and as the culprits of all the catastrophes, the players start at the wrong side and later they discover the are the ones threatening the world...
I've done something like that but with a scifi background behind fantasy, spirits, nature... all was created artificially by some beings who are plotting against each other, spells are formulas to control the nanomachines and elements, etc. I always liked the anime of "the scrapped princess" and the videogame "Might & Magic world of Xeen", so I used those elements for an alternate Rokugan in L5R, first the players were getting small details, for example in one adventure they had to find a mysterious orb from the past before other parties, it had a strange language but deciphering it was another adventure, later it exploded and when they could study the writtings they saw it was a bomb, it worked well when the players each adventure discovered a bit more of what was happening beyond the world they knew, machines that transformed people on demons/onis, a virus in a central computer of a region that tainted nature and people, etc.