I find magic to be a really interesting part about COE, seeing as it would only affect 5% of players. Here are some of the ideas I've had in my head about magic
1) I really like the idea of player-made spells. I think it would be a waste to have pre-made spells seeing as how it would only cater for a minority of the playerbase, however I think it would be quite difficult to code something like this, as not only would it be extremely difficult to do, but it may make it heavily unbalanced. In this case I think the devs should make it so that they have set rules on what magical limitations there are.
2) I don't think that there should be something like a mana bar since a lot of other mechanics in the game are represented as vitality; health, fatigue, hunger etc. I think that depending on what kind of spell the player casts, it would affect the player in some way, be it making them tired to some degree or physically injuring them dependent on the skill of the caster and the difficultly of the magic cast.
3) There should also be something like a skill range on how easy/difficult players find. For example, some people may have similar spells to one another, but one of them may find it extremely easy to cast, requiring only little effort and almost no affect on themselves, whereas another may find the action tiring and time consuming. Seeing as magic is a talent, I would think that players with magical affinity would also have talents in different areas of magic.
4) There could also be other limitations, such as a player only having affinity for casting one spell but being exteremely adept, or another being able to cast many different spells.
5) The game could have different trees of magic which can be studied; illusion, creation, destruction, healing etc, which players could be skilled in one or multiple trees. There could also just be trees which are similar to one's most preferred so they could find it easier to pick up.
These are just some of the thoughts of my half asleep mind, I think I may have forgotten a few that whizzed through my head