It's an interesting question with this kind of ability whether the guidance such an additional clause is helpful enough to warrant the cost of extra text.

I don't worry too much about the complexity it adds, but also don't mind the minimal usefulness of activating the ability in response to e. g. a surprise blocker with flash.

I feel I'd rather save some space in the text box.

Dunno if this neither here or there, but would adding a "Activate only as a sorcery" at the end there be preferable? Activating it on your opponents' turns certainly doesn't make much sense.

Namely that being able to activate this at instant-speed might give the impression that doing that once the creature has already declared blockers assigned to it would do something.

Sorcery-speed might also lessen potential on-board complexity concerns (which could be notable for a common) if with instant-speed you could have some extra evaluations of mana usage with this ability along combat tricks or so on. Albeit those seem like they would require very specific conditions.

Aye, yeah, I think it almost ought to cost 1 mana these days. A hybrid Healer's Hawk seems sensible considering that even black has numerous 1/1 fliers for 1 with additional positive abilities. For example, Blight Keeper, Battlefly Swarm, Eye Collector, Eyetwitch, Faerie Dreamthief, etc.

See Bathe in Light, Boros Charm.

...

See Wojek Siren, Stirring Address.

aka Moving Monologue

There is no way to make this cheap enough. :)

See Cleansing Beam.

• Radiance is not a good mechanic. You know what is better and actually works like Radiance? Overload. Also has the same restriction to targeting spells.
• Between Electrickery and Mizzium Mortars.
• I prefer "an opponent controls" over "you don't control" here

In another time-line this is strive, but I'd like to vent on how a different guild snatched the proper implementation of radiance on the return.

See Rally the Righteous.

• Radiance is not a good mechanic. You know what is better and actually works like Radiance? Overload. Also has the same restriction to targeting spells.
• Loosely balanced around Dynacharge.

In another time-line this is strive, but I'd like to vent on how a different guild snatched the ptoper implementation of radiance on the return.

This is hurt by being gold. Untapping is not worth the increased mana value over Dynacharge.

Compare Acrobatic Leap, Arachnoid Adaptation, Gerard's Command etc. Tempted to add reach.

See Gruul Nodorog.

• was too expensive compared to Falkenrath Celebrants, Shatterskull Recruit, Granitic Titan, Ripscale Raptor; green gets 4/4 with upside at ...

See Mourning Thrull.

• Between Healer's Hawk (Ruin-Lurker Bat) and Daggerdome Imp (Deep-Cavern Bat)

In legacy-oriented world tempted to cost this at since the power has leapt.

See Gaze of the Gorgon.

• Regenerate -> indestructible UEOT
• triggered basilisk ability -> deathtouch
• -> (closer to [[[Offer Immortality etc.)

This is a ten-card cycle of hybrid commons. Outside of that the name gives kindred vibes.