You say that you want to make a different game and a different experience, so my ideas are coming from there, you want different then you have to break stereotypes, like this one where the priest is always the healer/support and warrior is always the tank, and so on.
In life there's always more than 1 way to do things, so what can be better in an MMORPG than having a lot of options to heal, to tank, to support and to doing damage?
I see Paladin and priests as 2 specializations of the same class.
If you don't want people to see priest as the always light armored with staff character and Paladin as the two handed and heavy armored character you can maybe merge this 2 classes in one called maybe Crusader, Templar, Apostle, Holy Saint/knight, Light Bringer/Apostle.
This way you can include skills trees for tanking, dealing physical and magic damage and healing/support, and depending the way you are building your "Holy Saint" you have to use the armor needed for that role.
Plus each type of armor with their own skill trees/passive skills, I like more the passive skills, for example light armor