For all we know competitive multiplayer is implemented via in-world arenas with a variety of team and map sizes.
If they go the in-game arena route for the competitive multiplayer, they could have some arenas that allow customizable gear and others that don't.
Oh, you'll find I'm the very last person to hate on Activision. I'm saying they very likely have as much interest in emulating a title that niche as they do in publishing one: None.
It's Bungie's game, but Acti is still the publisher which means there's a level of compromise going on. Dark Souls and the things that people appreciate about it most are not palatable to a mainstream audience and those are the only games Acti deals in.
For a supposedly niche title, Dark Souls sold remarkably well. Activision may want to emulate its model more than you might think.
When I talk about Dark Souls being a model for Destiny, I'm talking about the inter-connected world and the online play (co-op and competitive) being fully integrated into the campaign, not necessarily the soul-crushing difficulty or super-enigmatic storytelling.
But knowing Bungie, I'm sure there will be plenty of options for those of us who prefer masochistic levels of difficulty.