1. It is not perfect, because too many people are complaining
2. Can't increase playing cost much, because it would make it to be useful only in rainbows
3. Don't like adding activation cost, since it makes that Shard and its use even more complicated
4. Fever turns when it comes to Black Hole - I agree with that, a simple and sensible nerf.
5. Freezes/ rewinds rather than destroys - an interesting idea for a new water or time card
6. Lower HP if turns into Black Hole if mark is not gravity - a possible way to nerf it, but in order to be a noticable nerf there should be too big differences between Shards that are played with mark and that are not (and are therefore destroyable with the usual CC).
7. If HP falls rather than rises then it is a buff not a nerf
8. Using a Shard too much in a row creates a Singularity - no more Singularities (read: too coplex and artificial nerfs), please!
9. Introducing more anti-PC counters. Something like PA for aether or entropy, for example? Not sure. Basically that kind of indirect nerfing is a good idea, because creates new cards, but that takes a long time and needs some deep thinking and calculations
10. If it doesn't creat BH then it should be a fire Shard
11. It shouldn't be gravity pullable - actually I would be glad if someone uses it like a slower version of an Armagio. No need to take away that kind of secondary functions that enrich the game but still do no make SoF considerably stronger.
12. No reason why Pillars, Pendulums or whatever should be untouchable if they are not immortal cards themslves (like a Mirror Shield)
13. If it requires gravity to use then it wouldn't be a Shard. Now if there are new permanents and Shards added there just should be an option for every element to destroy those, otherwise stronger elements (having PC)just becoming stronger and weaker are becoming weaker and too dependent on other elements that have PC.
14. Activation costs grows - I didn't like adding activation cost
15. Change it profoundly - no, that Shard is needed because many perms were added
16. Other nerf - not necessary, I already agreed with the option 4.