Just trying to brainstorm some thought based on what this class is supposed to represent, and what I have enjoyed from previous MMOs.
I think a Priest should have some holy type spells, in a similar style of what a Paladin would have, but they should also be clearly differentiable. Priest would probably have spells and effects that seems more (multi)religious, and not purely about light. It should definitely have some type of cleansing or healing spell that can remove bad effects, recover people, purify them. I feel like a Priest is supposed to clean, remove, counter everything that is negative and dark, but maybe it could also have spells that just contains or traps this darkness, like a holy jail.
Since a priest would work against and counter darkness, it would also probably have a lot of knowledge about darkness, which could result in a specialization or sub-class of Priest that is completely opposite of what I described above. A dark, unholy, cursed priest that would draw power from others, apply bad effect, resurrect creatures as zombie like to fight for them.