As long-time readers know, I am a proponent of experiments, of trying new ideas out and seeing what problems result, rather than trying to predict everything that will happen in advance. The best knowledge comes from previous experiments. We know unlimited PvP is not popular, because of the Felucca/Trammel split in Ultima Online.
There are lots of experiments I would like to see MMO companies try, even if most of us think they would end badly. Of course, it's hard for an MMO to wildly experiment when the cost of a failed experiment is the loss of many subscribers. I'm sure Blizzard has regrets about some of the experiments they carried out in Cataclysm.
One experiment I would like to see is the release of new content without increasing item level. A new raid tier is released, with new loot, new item sets, but the exact same item level as the previous tier.
Now, I think that we're all in agreement that this would go over like a lead balloon. But none of us are absolutely sure. It's possible that the audience would be fine with it, maybe for a little bit.
The thing is that if the audience is okay with new content at the same item level, that would help enormously with some problems in endgame. You wouldn't have this constant ramp up of power levels. You could introduce more tiers, or space out content in a cleaner fashion, without impacting the power level. You wouldn't need as many catch-up mechanisms.
Admittedly, it's unlikely that keeping the item level the same would be popular. But we don't know for certain.
Another experiment I'd like to see is not having raid content available on release. Open it up three months afterwards, and make sure that everyone knows this is the plan. I think there is a much greater pressure to blow through leveling content among veteran MMO players. Perhaps giving those players time to digest the leveling experience, to stop and smell the roses, would be beneficial in the long run.
What experiments would you like to see an MMO try?