Ideally, shard of focus should be comparable in power to something like butterfly effect. BE is the weakest form of PC currently ingame, and an other card accessible to any element should be among the weakest, not the best one.
That is an interesting idea. SoFo could be a spell that transmutes a creature into what the current shard of focus is (like mutation). It should be a new creature so you can't use SoFo's ability on the turn you play it. This would also make the card a form of CC.
Actually, that post just gave me a GREAT idea. Turn shard of focus into something like this...
Cost = 7 // 6 (like now)
Type: Spell (plays on a creature, like butterfly effect, blessing, etc)
Text: Target creature gains the ability Accretion: Destroy target permanent. This creature loses 10hp from its max hp, and this damage cannot be healed. Turns into a black hole if it dies by this effect.
This would, on most creatures, result in destroying a single permanent. On some dragons, like upped light dragon or upped earth dragon or steam machine, you could do it twice. On gravity creatures, like massive dragon or armagio, you could do it three times (and thus increase yet more the gravity synergy) The no healing clause is to prevent stuff like holy flash or archangel turning it into the same near-perpetual destruction machine that gravity force can do to it now. This could be a little too abusive with voodoo dolls, but even there, you'd do 10 damage one time (the second time, the doll dies and thus does no damage) so it's probably okay, and you'd have to use deck slots on dolls which are kinda weak anyway, whereas a card like gravity force is useful in other ways, like CC, on your own other creatures in an emergency, etc. Also note that this card won't be broken by phoenix, since when the phoenix dies, it wouldn't have the original phoenix ability, as it is replaced by accretion; same thing with ash and the rebirth ability. It would be like how you can stop phoenix/ash now by liquid shadow or overdrive, or stop ash rebirth by butterfly effect.
Thematically, I'd argue that the creature is losing mass to the accretion ability (rather than the way the shard is gaining mass) so that hopefully justifies the reversal from hp gain to hp loss under this design from a thematic point of view. It also makes the shard easier to deal with by CC. The one buff that would actually come about is that if the creature were already in play, you could theoretically play SoF on it and activate it right away (like how you can do with butterfly effect, assuming sufficient entropy quanta) but I don't think that's too much of a problem, given that outside of gravity it's capped at 2 activations anyway, and will usually be one.