Targeted ethereals like the WDs that specify a group of skills can make old builds or skills fun. Some like secondary on necro already have a weapon boosting them but one for blood skills or corpse ones would make for interesting choices.
Drop only. Having that one or two item type to hunt in game for is amazing. It helps more on console to prevent save state scumming and shinanagins.
Rewards for collecting, easy win
If ethereals are to be off season the flat damage bonuses would need to go. Having more stats amd a random legendary power makes them better already. Throw in the ability to reroll one stat perhaps with dust from other ethereal and you can craft a near perfect one in a season.
More of them would be needed for the builds that got excluded like shadow set on dh.
Adding more to the pool has a downside of making RnG worse but drop rate can be adjusted accordingly to 2% or 3%.
Whatever the solution is they should be kept drop only.