Inelegant Design
It is difficult to say whether the unstackability has been factored into the cost with shields, as there are rarely any other (stackable) cards with similar effects to a shield.
However, it seems certain to me that unstackability has at least been factored into the cost of weapons. Weapons generally have a very low cost in relation to their power, and this is balanced by the player's inability to stack them (Flying Weapons involve additional costs in quanta, probability, deck variety/size, and number of turns).
I assume that other unstackable permanents have been balanced in a similar way, that the possibility of a dead draw has already lowered their costs.
I support not changing them.
Despite my position, however, I do have some
Ideas
... if a change must be effected.
The flooding mechanism sounds like a workable idea. Though it now seems more like a demonic gate / dark channeling than a nightfall (unless the stage of battle includes multiple moons and stars for multiple Eclipses and nights)
Alternatively, a drain-all-mana mechanism like the one in use for Sky Blitz and Fractal could be considered.
I have not thought about any of these in depth - merely throwing ideas around
More radical changes may include adding negative effects to the card:
Ex.1:
Remove all active Darkness / Death creature abilities and unburrow all burrowed Darkness creatures when a second Nightfall is played. This one may see coding issues, but would introduce new strategic elements to Darkness / Death plays.
Ex.2:
(For each Nightfall) Increase attack, but decrease defence (+1/-1) for Darkness creatures; increase attack without affecting defence for Death creatures. This is far more significant for Darkness than a first glance may suggest, as a (+1/-1) Nightfall would:
- bring an Obsidian Dragon into Lightning range
- fail to boost a Black Dragon beyond Lightning range
- be unable to double-boost Devourers and Minor Vampires
Summary
While some ideas to make Nightfall stackable may seem pretty interesting, I do not think change is necessary.
Perhaps a new card that acts as stackable Nightfall, but with a negative effect. Choices for the players