currently, the ghost of the past is the only card which gives a penalty or reward based upon discarding from the hand. it may take some tinkering to even come up with a reason to reward someone for discarding their own card, as i havent been able to come up with a valid reason for implementing such a mechanic. so instead, continuing the "you dont want to discard this card" ghost mechanic could be expanded to other elements, and there could be other ways besides nightmare to get those cards into your opponents hands. other potential ideas include:
add poison counters to owner
have a black hole effect applied to owner
opponent healed 20 health
pandamonium owners creatures
destroy a permanent of the owner
earthquake on a pillar of the owner
place a copy of the ghost into play for the opponent
(the quanta cost, attack and health for each card can be adjusted independently to maintain overall balance)
cant think of anything for 
or 
, so they may be good candidates for the elements to get cards providing a similar effect to nightmare.
I do like the Ghost of the Depths, but I have a few suggestions on the others:
Kill random creature owner controls and replace it with a skeleton.
I feel like this one should be the Poison spawner.
............I had an epic idea for this one, but now I can't remember.
I think it should deny you the abilty to attack. Kind of like a D-Sheild but only for one/2(upgraded) turn(s).
I think this should be the healing one, as there is really nothing else it could be. Also I feel it should be 10 not 20.
only does 10 damage so
should only heal 10.
Maybe double you opponents airbourne creature's attack for one turn?
*EDIT*
I have a thematic issue regarding this "ghost" series.
The theme of "Ghost of the Past" is that the past, or history, maybe a horrifying memory, is haunting you like a ghost. This is also why the ability of GotP is called "Obssession", something from your past is haunting you and you can't let it go. That kind of negative energy eventually becomes a creature.
But if we extend the "ghost" to other elements, will this thematic meaning of GotP be lost ?
No, I don't think the meaning of ghost of the past is going to change. I think by expanding on the Ghost of the Past's elements it would make it more "Haunting." While I do agree, that to keep the meaning, there might be a name change in order for the Ghost of the Depths, (Maybe Ghost of the Well) and possible a slight change to make the effect more harmful to the target and less helpful to the one that initially used it. But the idea is still the same.
