Today, mobs often take way shorter time to kill than in older times. This interferes with dot classes, whom ramp dps up over time. Also, it changes the playstyle a bit. Doubling(tripling?) trash mobs hp (and xp reward) compared to player dps in new expansions would change this, I assume. It would be a change, so it would have to be communicated etc, and it could be spread over several expansions. The risk is that people that are slow killers today, would be ultra slow killers. But maybe segregating slow kill mobs to a few zones (in a new expansion) could be a solution, and also provide experience for dev team, player base, and community. Would you like slower kills?