I like the idea of giving druids a thematic but distinct way to be able to add (elemental) damage to melee. The model here is astrologians, which also don't get the usual spells (mw, flameweapon/darkfire), but have stellar blade which functions uniquely for that class and yet fills the same role.
Suggestion: Lower Herald of Winter from circle 6 to circle 5 (this would match its innate spell circle) so that this is an achievable spell to cast. As an added benefit, it puts an ice/cold theme spell at circles 2-5.
Alternatively, add lightening avatar to their spell list (either at its innate circle of 4, or at 5). This would round out a lightening evo focused druid to something to use beyond call lightening and I think is appropriately druid-master-of-the elements themed.