I think the problem with current nightmare is that it does'nt do any harm to the opponent. Filling empty spaces in your hand with cards is not a problem unless one is using a Fractal deck.
So the problem with the card is that,
1.) It has a very weak effect.
2.) It lacks flexibility.
My opinion is that adding two effects to a single card is not a good idea unless it's a unique one which fit the theme of the card and i think the easy best way to go would be a total remake of this card into something more devastating and flexible.
A possible suggestion which i am not confident with, is making it into a strong high cost card which drains all the darkness quanta (similar to miracle and sky blitz) and replaces the opponents entire hand with targeted creatures card (not just filling empty spaces in hand but the entire hand itself).
This would be very powerful as it makes opponent lose his all cards in hand but it is justified by a higher cost and by draining all the darkness quanta. It can be countered by using all the cards as fast as possible but if he/she does that then more of the opponents hand would filled with more creatures (self synergy) and makes an even stronger combo with GotP. One fact that will not make it overpowered is that the repeated use of such a card does not do much. It would fit the theme and add a new card for darkness in the quanta drain series.