Dynamic events/puzzles in a cave is what I thought of as well when I watched that, rather than something new/ANet forgot to mention. Probably as close as it will get with it being in the open world. With multiple groups in a single dungeon it would turn into raiding(I seem to recall reading somewhere that ANet wanted to avoid large numbers in a dungeon due to profession stacking for X skill, and that personal effort would have less impact), maybe if there was a limit of 3 groups per Explorable dungeon with each one being restricted to one of the pathways.
But what would be the point of going in with multiple groups if you are separated? If it is to race the other groups for 1st, 2nd, and 3rd(with useful/shiny items awarded) it will be abused by 1-5 players bribing the other group(s), which would diminish the value of the rewards as anyone with enough gems/gold can buy it when that was not necessarily intended.