Some background first. I've played DDO for a couple of years. I have a TR1 wiz sitting at level 24 and a couple of alts mid levels. I play a few hours a week and I can afford to spend cash on the game should I want to. I am in a good but small guild and play European time so I PUG or solo exclusively. I am a little bored with the game.
I like PUGing, I like that this is a social game with some great people playing it and that I enjoy grouping with, however, given that I am usually on a time constraint it is not easy to find a group for a quest you want to do quickly. This is especially true at the higher heroic levels (16-20) when even an hour is not enough time to find a group and complete a dungeon during the hours I play. I do not think this sort of time constraint exists so much at lower levels so I would recommend this idea be aimed at levels 16-20.
I suggest a new type of adventure pack, the "come and go". It would have a setting e.g. forest or devils battleground or whatever and basically one path through that setting. You would have a large number of encounters in the pack and as you wander along the path you would randomly spawn one of these. Most would be beat up the monster type encounters, some would be protect the villagers, some would be puzzle based etc. With a small number of models you could easily make hundreds of encounters.
If you wanted to run a "come and go" adventure you would go to the quest giver and you would enter a queue. If a space was available in an existing group you get parachuted in, if not you enter a queue and wait until either a space is available or there a 4 or so people in the queue to form a new group.
Once in a group there a 2 statuses: green, when no encounter is going on and red, during an encounter. You can leave the group at any point when the status is green and each encounter is short so you can literally "come and go" as you please in this pack. Once you lose a player from your group a slot opens up for anyone in the queue. If two groups get too small and there is no one queueing the groups get merged (think tables in a poker tournament).
Now this would be pure PUGing and you would have no idea about the skill or ability of the people you are going to adventure with and you don't know what is coming next so XP/min would have to be pretty high and you would need to be pretty self sufficient to survive. I propose when a player leaves you get to rate them (thumbs up/down/neutral) and that over time determines their reputation. High rep players get put in groups with other high rep players and vice versa. If you get your rep above a certain level you can apply to become a "sensei" and you get dumped into the worst groups and teach them how to play in exchange for a massive boost to your XP (thinking +100%).
This whole situation would work best when there is a large pool of players obviously. I would also set up a forum for encounter suggestions from the players - it costs nothing and there will be some great ideas.
This would be great for the casuals who play on a tight time schedule and often have to leave quickly because a (usually family) situation arises. It would lets us dip in and out of the game easily and let us meet a large range of people to have fun with and after all we play this game to have fun not wait around for a group. Ever played rush poker? - straight into the action.