Last Sunday a reader suggested making instances in World of Warcraft completely scaleable, so you could visit them alone or in a group of any size, and they would scale in challenge to meet the number of available players. City of Heroes works like that, so why not World of Warcraft?
I think the concept has two major problems: Trinity and loot tables. If you scale the difficulty of a dungeon based on the number of players entering, you still don't know much about the actual power of that group. In a group of 3 or more players, the power of the group goes up dramatically if there is a mix of tank, healer, and dps. Same number of players but missing one or two of these elements is much weaker. Thus the system would need to take that into account to work, and that would start to be really hard to calculate.
The loot tables problem is that if you give out the same loot for soloing than for doing a dungeon in a group, most players will just run it solo. Setting up a group is an additional effort, and that effort has been traditionally rewarded by WoW. The larger the number of players, the better the loot, up to the point where Naxx25 gives better loot than Naxx10, without always being harder.
Complete scalability will be hard to reach, but I would love to see a solo mode for all dungeons of level 1 to 70. I think soloing these would be preferable to the current practice of getting "boosted" through them by a high-level character.