When this creature leaves the battlefield, target opponent creates an X/X blue Orb creature token with flying, where X is the number of +1/+1 counters on this creature
Vexing Radgull Unless you care about radiation counters specifically, Vexing Radgull is actually a less reliable source of proliferation than Thrummingbird
There's just too much competition in the Human two-drop slot, with Thalia's Lieutenant and Thalia, Guardian of Thraben being auto-includes and cards like General's Enforcer, Seasoned Hallowblade, Kitesail Freebooter, and Meddling Mage also being strong options, depending on the build
#5 I find both of those to be upsides, honestly
Ambush Wolf Anara, Wolvid Familiar Arctic Wolves Arlinn's Wolf Ascendant Packleader Assembled Alphas Bounding Wolf Brazen Wolves Briarpack Alpha Bristlepack Sentry Carrion Howler Cemetery Prowler Darkthicket Wolf Deranged Whelp Dire Wolf Prowler Dire Wolves Ember-Eye Wolf Exuberant Wolfbear Fang of the Pack Fearless Pup Feral Ridgewolf Ferocious Pup Fierce Witchstalker Flourishing Hunter Heart Wolf Hollow Scavenger Hollowhenge Overlord Howlgeist Howling Wolf Howlpack Wolf Hungry Ridgewolf Immerwolf Kessig Wolf Lambholt Harrier Lightning Wolf Lone Wolf Lupine Prototype Nightpack Ambusher Nyxborn Wolf Pack Guardian Packsong Pup Pestilent Wolf Primal Adversary Primo, the Unbounded Pyreheart Wolf Quilled Wolf Rot Wolf Runebound Wolf Russet Wolves Sacred Wolf Sarulf's Packmate Sarulf, Realm Eater Scourge Wolf Silverfur Partisan Skalla Wolf Snarling Warg Snarling Wolf Somberwald Alpha Spectral Hunt-Caller Spirit of the Hunt Sporeback Wolf Summon: Fenrir Tel-Jilad Wolf Territorial Witchstalker Thornhide Wolves Timber Wolves Timberpack Wolf Tokka & Rahzar, Terrible Twos Tokka & Rahzar, Unsupervised Tome Shredder Torgal, A Fine Hound Tundra Wolves
The latter two in particular allow you to instantly convert a pile of random artifacts into lethal damage with almost no warning
