Hey Bulbasaur05! I love your idea of developing a Catholic MMORPG! Ill throw in my 2 cents and see what you think of it. An idea I had comes from what the Bible describes as the Spiritual War.
So for instance you would be an angel (with the ability to customize your appearance and clothing, maybe even wings) who starts out in Heaven. You find out that you have been created and St. Gabriel gives you a quick tutorial on how to move, fight, etc. then when you are ready, you enter into the main map/area which is Heaven, or part of it. Imagine the beautiful graphics and scenery that can be created. I would read the book of Revelation and the writings of Catholic Mystics who claim to have seen Heaven from a distance, Like St. Don Bosco, who saw silver tree trunks with golden leaves, lakes of diamonds and and golden streets and buildings, etc.
Also as I can see by your Avatar, you are a pokemon fan, so Imagine the creatures you could create that can be equipped and accompany the Angel on his or her journey. These heavenly creatures would have their own abilities, to help out when there are epic boss fights or something.
One benefit to making the character Angelic Is that you get rid of the whole “Magic Problem”. An angel can have lots of powers and abilities, but it cant be considered magic because, well its an angel. They are creatures of God and so all their powers and abilities come from God, depending on the task set before an angel. You could take inspiration from the Bible when it comes to creating the Angels powers, and then building on it. For example:
Gifts of healing (1 Cor. 12:9, 28; Acts 28:1-10)
Working of miracles (1 Cor. 12:10; Acts 6:8)
Prophecy (1 Cor. 12:10; 1 Thess. 5:20-21; Eph. 4:11)
Discerning of spirits (1 Cor. 12:10; Luke 8:29)
Tongues (1 Cor. 12:10; Acts 19:6)
Then again, the list of powers an Angel can have is limitless. You can create classes, ranks, and heavenly weaponry. Maybe as the angel LVLs up, it moves up the heavenly ladder of Angels. You could take inspiration from the 9 orders of angels for example.
Now there is the whole issue of questing. As you start out in Heaven, you would get quests from Saints and angels. Imagine St. Francis of Asissi in heaven, dressed in a golden robe with a halo around his head, under a silver tree surrounded by heavenly creatures entertaining him. You come up to him and he asks you to find a heavenly bird that went missing or something to that effect. You know, just something cute and innocent.
As the game progresses however, St. Micheal, the general of all Angels wishes to see you to inform you that Satan is at it again, trying to destroy the Kingdom of God, and you are to be deployed to earth (the “battle ground” so to speak (Also I think it would be cool if it was the middle ages) to fight “the devil and all the other evil spirits who prowl through the world seeking the ruin of man’s soul.” I can also see expansions in the future where you visit Purgatory and Hell, but that’s another story.
Now I don’t think there should be fighting with people, as this is a game about “Spiritual warfare”. You would fight demons and creatures of darkness. For ideas on how to create and design them, I would use the seven deadly sins for inspiration and build on that. I also don’t think they have to be too creepy looking, so you wont be accused of creating a “horror game”. Maybe it could be a little creepy, but with a bit of silliness attached for comical relief.
Finally, when it come to the fighting, I recommend you have a look at Anime MMORPGs like “Eden Eternal”. The world environments are usually very bright and cheery, and the fighting is not crazy and intense. There is no blood or gore. Its sort of Hack and slash, and then the enemy sort of vanishes. Maybe the demon could turn into a puff of smoke and then get sucked down to where it came from?
Anyways there is a lot more to this like the storyline, design and feel of the game. This is just some ideas that I have thrown out that have been rolling around in my head. So I hope it gives you some inspiration. I wish you the Best of Luck!