1. DRM are not a success. They are tedious and not fun. And no one does CMs after they made the achievements.
2. For me the instance should be like old fractals where if you pay attention its easy to do it. Not like the new fractal ones where damage will follow you no matter you will do.
3. Your idea can be good but not with harder mobs but with clever mobs, with clever mini events clever bosses where higher skills can make a difference. It's easy to deliver mobs with more HP and dmg and make him see you in stealth or make him curb his arrow and hit you every time, whatever side you dodge, but make the mob behave normal and do a challenged content is harder.
4. Rewards, fractals have also good rewards that's why peoples are farming them. Otherwise peoples will farm the achievements and forget about it like dungeons(some of them are really good), strikes or DRM. All this content wasted because Anet didn't think it thru.
5. For me the best way to do new instanced content in GW2 is do good fractals and create new rewards for them. Scrap all other instanced content dungeons, raids, strikes, DRMs , use what is good from them to make new fractals, be it mechanics, bosses or rewards.