I don't want to resurrect the discussion of whether we should outlevel enemies, so I'm basing this on the assumption that we will do so as we did in the old city.
If so, please make mobs that are worth zero xp not aggro as if they were non-greys. I'm playing Dragon Age Inquisition right now, and mobs that are too low level to grant any xp still aggro like level-appropriate mobs, forcing me to fight them for no gain. This would be just an annoyance if these 'grey' mobs died as quickly as they did in our old game, but they still take about the same time to defeat as mobs of my own level, and they can still kill my characters. If I have to put forth effort to defeat enemies -- and if they can do significant damage to me -- I'd think it's reasonable to expect to get xp from them.
Spurn all ye kindle.