Never should PvE and PvP mix. Division Dark Zone anyone? The single most frustrating thing for many players is being negatively effected by things they never even touch.
Division is just a shitty game with bunch of design choices that clearly were hastily thrown in just so the game can be shipped on time. That game was simply overhyped, and is poor example for anything.
Like I said FireFall was balancing for PvP for sometime, and it worked pretty well.
WoW recently implemented PvP templates based on class and specialization. You can run around in battlegrounds wearing nothing but a weapon and be as effective as the player next you in full gear.
WoW has to do that because it's stat base mmo, this is the only way they can make it work (and there are numerous mmos that have been doing this long before WoW too). They have to have something to equalize the power difference.
Ember is not going to be typical mmo. It will have horizontal progression, meaning that level 1 character is not going to be that far behind max level character.
So there is no point in making another layer of stats for pvp, because the nature of the game is already set everything to where players will be pretty close in power no matter how long they have been playing.
The only Real problem would be behavior of some skills. Obviously an AoE skill that can be cast by a single person and it's strong enough to incinerate large group of enemies, would be a problem when is used against other players, especially if there are say couple dozen of people using it at the same time.
So you simply have such skills do less damage, or have lesser effect when used on the playres. A stun shot can freeze a low grade AI for ten seconds, but it would only freeze a player for 2.
And that's how everything should be balanced, so skills would simply be less effective against real players, because we are staying omni frames that are protected against our own weapons. You wouldn't have to change between PvP or PvE sets, you just use whatever you want and learn how whatever equipment and abilities you are using behave when used against other players.
Also I just want to make it clear here that I am not asking for all out open world pvp, where people can get ganked and griefed. There has to be system in place that would prevent any of that, of course.
But my point is that, yes balancing pvp and pve in one world is difficult, but so as hundreds other things that comprise a successful game. Heck making a shooter mmo with horizontal progression sounds pretty tough, but here we are.