Are you going to make this purely based on TG or you gonna add some elements(ki, chakra, nen, etc) from other anime? If it's the former, maybe you can make him use 2 kagune since his soul is merging with kaneki's. Even though kagune have no relation with souls, just make up stories for that...If it's the latter, you can juat make him learn some moves(martial arts, cultivation, qinggong) to use RC cells in his body like intenal energy.