Mecha Gnar
I realize that this has been stated before, however I truly believe "Mecha Gnar", or maybe a "Neon Strike Gnar", or even a "Super Galaxy Gnar". The gimmick of this skin would be all in his passive. When he changed from his Gnar form into Mega Gnar, different pieces would then attach to him, creating Mega Gnar. The pieces would float/shoot from "nowhere", and create the bulk of his body.