In its current state, Shard of Void is seeing very little use in any sort of deck. Its application is similar to a 100% poison deck. Its advantage over poison is that it cannot be healed back, and it cannot be purified.
However, it suffers from its state of being a permanent, which allows it to be destroyed. But this is the least of its issues.
It has little to no synergy in elements. Its gradual but assured damage suggests it should be used in conjunction with stalling. Yet, its minimal damage and the necessity of protecting the shards by some means makes it unappealing in comparison to poison. SoV will do no more than 18 damage per turn. Poison can go beyond that if Arsenic or scorpions are used. (and they fit in with a stall deck just fine, due to SoSac, and not needing a permanent protector)
Furthermore, if damage is used in conjunction with SoV, SoV does little more than ensure that the damage sticks. It can act as a counter to healing decks. Then again, if the deck is using excessive healing, none of the damage from the creatures is sticking; just the SoV. So that doesn't really synergize. However, it does mean that eventually the creatures will be able to deal the opponent's new "maximum" hp in one turn. This means that the shard could be used to set up lower damage OTKs. But OTKs tend to be quick-draw based. They are also very chain-based, and drawing too many SoV's could wreck this chain. And fitting 100 hp into one turn isn't the fault of the OTK strategy anyways.
So then what CAN SoV be used for? It is harder to set up than poison, and has fewer uses in conjunction with other damage sources. If new cards are released that have interactions with the reduced maximum health, then SoV would be just fine. But in the current game, it is forced to be a stand-alone. And yet it is a very weak card, only dealing 3 damage per turn, which needs to be protected. Against False Gods you will deck out very often if you run 6 SoV as your damage source.
The way I see it, there are two different ways of handling this shard: bringing it into the realm of damage, where it can synergize with other damage, or strengthening it as a stand-alone.
If the shard reduces current and maximum health, it wouldn't be dead weight when used in conjunction with damage. When fighting against any deck without healing, the SoV would act as poison, and the creatures as damage. They would work in conjunction to bring the opponents current health to zero. And when fighting against a deck with excessive healing, the shards would allow the opponent's maximum health to be reduced to an OTK range for the user's creatures. The SoV may need to have a quanta cost nerf if this is the case, so that it is not strictly better than poison.
The other solution, increasing the shard's maximum health reduction, but also increasing the shard's cost, allows it to be used more effectively in a stall deck. It still would differ from poison stall decks in that the shard would be more quanta intensive, and require more cards to set up. (SoV stall would NOT work against the multitude of decks with explosion, SoF, steal, or pulverizor, if it did not include some sort of permanent protection.) It also will not work as well with SoSac, which is a major stall card. It would keep SoV as similar but different.