Ideas whose time has (long since) come
I'm deliberately going with the low-hanging fruit.
Craftable Paladin swords
Too slow to be unbalancing. If this is wrong, then adjust the damage. Won't diminish the artifacts with that graphic, because what makes them special is something other than the graphic. There are too few two-handed swords in this game that look like swords. Hell the EC mostly just uses the longsword graphic anyway.
Permanent Treasures of Tokuno
As far as I'm aware, the artifacts are useless to collectors but might be useful for new characters or players to actually use rather than sell. And those dyes of course by definition won't last forever. You also could trade out rewards (including dye colors) periodically.
Virtue Artifacts that Spawn Commonly
The whole point of these is for the post-New Haven crowd, wondering what comes next. That was a close paraphrase of what the devs at the time said when they were implemented. But they have always spawned far too rarely for that purpose. Fix that.
Radiant Quests
Skyrim-style. We're already getting this in the new shard. So why not for existing shards?
Non-timed Peerless keys
Doom gold skull style. A market could exist in selling keys.
Roof should work through keys, not access quests
Let some characters get access to the roof and then sell it in the form of the key, again like the Doom skulls.
None of the above should be controversial, and the inevitable "but what about"s are not really relevant.
Comments
Concerning the keys, I think both would be very welcome cause they address a larger issue within the PVM game. Most of the higher-end content is just annoying and time consuming to key up. Systems which let you key in advance/buy keys at your leisure are going to be more popular since they offer a lot of flexibility for different playstyles.