It's been quite a while, and not even a tease of any other "vaulted" Exotic Missions returning. They aren't supposed to be gone forever, and Bungie specifically mentioned that they would all be returning, and even teased a reprised Last Rites I believe, which wasn't even an exotic mission. Of course there was no exact timeline given on their returns, but I believe something like "every 3 seasons" was an approximation for how long it would take for them to rotate back into the game. If bloating the rotator playlist is a concern, there could always be 2 featured missions at a time, or we could remove some missions from rotation and make them permanent additions to the locations they take place in (though throttling the farmable loot and adding different avenues to earn Deepsights for people that need them). My only guess as to why most of these aren't back yet is that Bungie either doesn't want to make the Exotics Craftable yet, or doesn't want to introduce perks on them that are immediately powercrept by newer and better things. There's also the slight hitch of creating Master versions of these missions, and either fleshing them out some more or simplifying them down, such as will Chasm of Screams, which was just a strike with barely different mechanics.