just an idea, but maybe the player could get Spirit Tree blessing at 500 spirit light given (400 or 450 if 500 is used) to let them change to spirit form.
you can use the player side Character Customization to change your spirit form and can stay in spirit form all the time if you like, and you can mix the different spirit's parts to your own spirit form and can change your colors for the spirit form you like.
all scenes and games that show the player Character will show the player Character in spirit form and the player Character's spirit form will not have just a blank face.
you can wear what the spirits can wear in spirit form, but have to buy them and use them in your bag to do so. the outfit items will be removed from your bag and added to the player side Character Customization for spirit form use.
to change to and from spirit form, you'll have to ask the Spirit Tree to change your form.
just so that the spirits you've met know it's you, the spirit tree could tell them. on meeting you in spirit form for the 1ST time, the spirits you met befour your 1ST change will talk about your new form, and in spirit form, you will not be called a human, but a spiritual human.
alot to add i know, but i think that it be nice to beable to live more like the spirits. maybe even see the spirit's homes and have a spirit form home of your own that you can make changes to. maybe the spirit tree will let you climb up it in spirit form only.
at sometime, job requests from the spirit tree could be added, as a new way to get spirit light, but you'll have to ask the tree if there is something you can do, but it'll not always be a yes and you can only ask in spirit form.