If she comes down at T3, might be too much bookkeeping before the creature type changes become relevant at turn 6. As abilities go though, "gain control of all Zerg" sums up what she does in the whole of HoTS.

Turned the three heroes into planeswalkers, together with (((Raynor, Rebel Commander))) and (((Zeratul, Nezarim Prelate))). Kerrigan is strange that she doesn't really have an "ultimate", in the strictest sense of the word.

I find it really nice, though, how her last ability summarizes her entire campaign in Heart of the Swarm.

Playtest update: I'm satisfied with her power level here. The versatility of her -2 is what makes the card. Turning Vipers and Scourge and Forests to 3/3 flying monstrosities is good, shrinking Archons is also good, and stealing said Archon is awesome.

Previously:

Kerrigan, Queen of Blades
Planeswalker - Kerrigan [M]
+1: Target permanent becomes a Zerg creature with base power and toughness 3/3 in addition to its other types.
-3: Destroy target creature.
-7: Gain control of all Zerg, then each opponent sacrifices a permanent for each Zerg you control.
4

• The first ability is a bit complex. stacking multiple permanent types isn't exactly newbie-friendly, and the "base power and toughness" is rare enough that I'd rather remove it.
• Shifted the first ability to the -2, and changed it to a sorcery Beast Within (Zerg within?)
• Observe first ability carefully to see whether it's a runaway win with an unblocked flyer.