Perhaps Lamb and Wolf take the form that relates to the origin of the one they hunt. For example, if they were hunting Warwick, they might take on a Zaun-related theme, allowing them to exact their toll as the reapers of their target's home town. Just a few suggestions:
Shurima: Lamb wrapped in sand-coloured cloth, obscuring her body and her mouth and leaving her golden eyes to stare out at people. Wolf becomes a predator of the sands; leaving behind a trail of sand as he travels to consume those who refuse to accept the desert as their tomb.
Piltover: Armed with a bow of hextech origins and covered in a cloak woven from magic-infused fibres, Lamb seeks to track her prey down while Wolf tracks them as he leaves a trail of hextech magic in his path: They are the true law of this city, a law between life and death.
Zaun: Radiating a green-toxic energy, Lamb seeks to grant a swift death, using a metal compound bow infused with toxic properites, from the endless radiation that would kill someone slowly. Wolf revels in the slow death of radiation, and emanates a dark, green aura as he hunt his prey, leaving behind a trail of smog to follow him.
Freljord: With a bow made out of the same true ice native to the region, Lamb hunts her prey dressed in a white/light blue snow outfit while wolf travels underneath the ice, waiting to crash through the ice and take an enemy to their freezing death as he leaves behind a trail of ice and snowflakes.
Some quick others:
Demacia: Light-themed, seeking justice.
Noxus: Dark/Evil-themed, seeking vengeance.
Void: Dark Purple themed, seeking to drag souls to the void as payment.
Bilgewater: Blue-Green themed, involving sea-themed ability models, seeks to drag enemies to the deep to their watery grave.
Bandle City: Could be similar to the Elderwood skin bard possesses. I have seen an Elderwood Kindred concept, and it looked amazing. Perhaps check it out if you haven't already, it can be found under the "Best" tab of Concepts and Creations.
Shadow Isles: Another dark theme but tinged with a white/blue energy by both lamb and wolf. Wolf would leave behind a trail of black smoke and white energy, while Lamb hunted enemies with arrows made out of pure spiritual energy found only on the shadow isles.
A Theme For Those Who Do Not Belong To One Region, Such as Kayle and Morgana: Dressed up in travellers garb, they appear to blend in with the others who travel the roads of Runeterra. Lamb, armed with her oak bow and "iron" arrows while hiding behind a wooden mask, seeks enemies across the continent to bring swiftly to their deaths. For those who run, Wolf will chase by any means. As a denizen of the forest, Wolf's visage could be one of a bark mask with sharp teeth, leaving behind a trail of green/brown leaves wherever he strays. The wooden mask on Lamb also fits the Kindred's theme by resembling a travelling Bard who uses masks to put on performances - found in her lore is a story about her hunt for performers such as those.
