Larian Studios Forums Divinity - Original Sin Divinity - Original Sin - General PvP arena mode

You're talking about two completely different approaches of the game. The whole game is not designed to be played with strangers. There are no lobbies, no rosters/ladders/match-making, etc... And so far we can assume there won't be anything of that sort. And personally, considering the philosophy of the game, it's perfect.
Now, if because of the PvP feature (which will be merely but a filler for something that has never been intended), Larian has to redesign the accessibility of the game, then I'm clearly against the idea.

Now I think you underestimate the power of modding. Not only do we have a basis on which to work on (friendly fire), but also will we have an editor shipped with the game, for free, and a massive bunch of possibilities with just this editor. But is that to say we're limited to the editor ? Clearly no.
There are many other ways to mod a game.

The PvP is likely to be modded. Now the question is : Will people be interested enough in PvP to install the required mod ? If not, then I don't see why Larian should bother working extra hours for something that's not particularly wanted. If yes, then they will be served.
As for PvPing with stranger, I understand the difficulty of organizing something with someone you don't know, but it's not an impossible task. As to how will it be done, I have no idea : Forums ? Groups ?
Regarding the fact that every strangers should have the same mods installed, it's irrelevant provided you can organize yourself, on the model I stated above.
If a group form and want to organize a "Deathmatch day", let's say every Sundays, I suppose they'll be organized enough to indicate which mods are required. Common sense.

As for all these rosters, ladders, balancing and all, I'm sincerely against it as far as the vanilla game is concerned. The game is not revolving around the idea of competition/e-sport, but rather about having fun with friends. You don't need all these features to experience it.

If the interest in PvP is big enough, maybe Larian could design something AFTER the release to facilitate the organization, but I'd honestly be pissed off if they waste their time on something that hasn't been thought of from the beginning and that isn't even assured to be popular.
That would be a sad waste of energy.
As I said, Larian has shown great expectations for D:OS, I think they have much higher priorities, and PvP is not something to be designed in a week and thrown away at fans.