thunder? i think? because after fuoco its thunder right it connects da fuoco to thunder its like one of the four great elements and one of the best legends.. so for me it's thunder o electricity something but if his alice would change i probably say that its thunder.. it even still looks cool to him when he use fuoco and i wish to see him using thunder.. i really still wish that there could be season 2 for gakuen alice :( that's all thank te for Leggere and understanding...
*SPOILER ALERT* Natsume originally inherited his father's alice of fire. So I personally believe he woulf inherit his mother's alice of memory recollection, where she could see the memory of any living thing. I mean he inherited her alice life-span.