I love the idea of hand-painted walls. They are so unique and almost any kind of design can be achieved. Or you could have a printed removable mural as down below from Ecosalon.
above 3: de Gournay
photo: Rachel Smith - Stockwell house (spot the mural in the hallway)
Robert Lenkiewicz's Riddle Mural in Port Eliot house of St Germans
Ecosalon - printed removable mural
above 2: Paulin Paris