Priest doesn't fit DA lore. There are no Gods that grant divine power like DnD. The priest archetype is filled by a support mage (Probably a KE with Spirit tree maxed, barriers and dispels, sadly DAI lacked in the buffs department). There isn't really room for a priest archetype who doesn't get their power from the fade, and if they do get their power from the fade, well they are mages, and why would two different mages have completely non-intersecting skill choices? It doesn't make sense.
The warrior/rogue distinction is already slim enough.
There still needs to be a solution for the combat and class system of DA, which is one of its weak points.
3 classes is bad enough as it is, with only 3 specializations per class you can't represent archetypes like Druids, Clerics (aka shapeshifters or spellswords) and all the others with just these. Unless we're back into "Mages are just D&D Wizards", which is just limiting and unfair imo.
As for Lore, i can't really respect lore that is there just to limit you.
Bioware was bold with Mages as they are, but it will always limit them in how they approach their class system.
If they can't escape from it, and bring a fourth archetype, then they need to allow mixes of the existing 3, like it happened in Mass Effect.
However, there is a solution for the Priest class. It doesn't have to be thematically like in D&D, only mechanically. They are still Mages, but they are trained in arms and armor as well, and thus have more limiting magic, and not blast type, like the typical Mages.
Imagination is the only limit. But i don't see too much of it, to be frank, at least for the class system.
You can't expect to represent everything, or at least a lot of things with 3 classes and 3 archetypes each, and Mages that resort to just elemental magic. And i don't think it's fair to ask people to support a game that doesn't offer them their favorite archetype, especially when it's a core archetype of every RPG out there that deals with fantasy.
Mechanically, Cleric was the 4rth archetype always, even with the 0 edition D&D.
You can ditch it, but will you still represent it some other way? Or will people be forced to play something they don't particularly enjoy? Or just hope what they want to play is in the game? Just for the sake of your "lore".
I am not really obsessed with Dragon Age lore, so i can't just enjoy its story only, or its interactions. It is still and RPG, and the RPG elements need to allow for diversity in how you want your character to play.
I know there's more to a Bioware game than combat and class mechanics, but the experience suffers a bit.
I don't necessarily want the Cleric per se, but gimme the option to make that Warrior Mage, without forcing everyone to lose a specialization slot because of me.
Knight Enchanter did the trick somewhat in Inquisition, not perfect, but ok, will we always need to close a specialization slot for melee mages from now on?
I think there's a better solution.