I honestly think this could be fun, if done right. I am a bit biased when it comes to pvp so please keep that in mind. Imagine, in order to get to certain dungeons (and/or raids) you had to wait until the gates, or entrance opened. Maybe a daily reset or a serverwide effort to achieve. Once the gates open, each group would have to battle their way to the day/week). Make the prizes and rewards worth it, and make old content worth going through again (for higher level characters give them a heroic mode or debuff).
Sure, there will be players that "learn the system" and yes they could add randomness to the whole ordeal. Knowing that eventually guilds and specific groups would take over the scene, I have a few ideas.
1. Put a timer on how long characters can be at the entrance. The more kills, guild/party members, and how big of a level difference could factor into how short they can stay there. Once that timer is up, move them away, and in some cases even phase them out so they can't keep harrasing others past the mobs.
2. Once you/others have completed a dungeon or raid for the first time each week, a few things could happen. Once you come back, you'll be phased and have to battle through everyone else who has completed it in addition to a possible fresh battle to/from the entrance. A different approach would be to instead make it so that you couldn't harm others who haven't completed the dungeon yet, and visa-versa.
Sorry about the rambling, but it seemed intriguing so I had to type it. I personally wouldn't want to see full on pvp dungeons, maybe group vs group or raid vs raid could be fun. Add a ranking system, RNG mechanics, and surprise (like trapped doors, levers, spikes, poisons, turrets, etc) mechanics. I honestly would find it hilarious being able to disrupt the enemy group by poisoning or slowing them down (cobwebs?) While also trying to keep my team alive from their side of things. After, I could see a fun battle to the death for loot.