Honestly, if there were to be some kind of difficulty bar to aim for, I'd name the following dungeons for MSQ difficulty: Post-nerf Vault, Sohr Khai, Baelsar's Wall, and The Burn.

For optional dungeons, maybe just a slight step above The Burn to pre-nerf Vault, or hell, even a lvl 80 version of pre-nerf Pharos Sirius and Aurum Vale, but scaled to appropriate levels.

I don't think that would be too difficult for the average-skilled player, would it?