This. Though some of the content is more then hard enough.They just need fights to have intrusting mechanics you cant sleep through.
They would also have to create/add
1: New weapons. Whip, 2 handed axe, pole arm etc.
2: New utility skills & elite skills. While your at it rework some of the existing ones to be more in balance vs the meta builds.
3: Expand on the trait system & bring more of the existing ones up to where the meta builds are balance wise.
4: Spread more of the existing weapons around to the existing classes. (thief with off hand sword & longbow, guardian with warhorn & axe, Mesmer with dagger & warhorn etc....)
5: Make world boss fights instanced with the difficulty & rewards going up with the more people are brought in. This would allow for very hard content but make it so idiots would be less likely to screw it up for everyone else.
6: Bring back Cantha & or Elonia & give us some story characters that are written well.
7: Last but not least make bosses that not only use intrusting mechanics but block, dodge etc.. so they are more then damage sponges.