If a namikaze uses reaper death seal on an uzumaki they become menma. Similar to how uchihas become obito when hatake steals their eye. You could tweak their rasengan, give them Dark Kyuubi Chakra Mode but they cant get Sage Mode. Instead they could get 1 regular sharingan but not ms. Their Dark Kyuubi cannot be stolen.
-Taijutsu
Kama Throw
-Ninjutsu
Rasen Ring (Rasenringu)
A variation of the rasengan. Dark purple with multiple white rings around it and is short range like the rasengan.
Great Rasen Ring (Dai Rasenringu)
A variation of the rasen ring. Dark Purple with a one white ring around it and is thrown like a rasen shuriken.
Repulsion technique
Fires a repelling force forward with a 2 tile width and has a range of 5 tiles. Does mild damage.
-Game Secret
Menmas don't get Ashura Mode. Instead they get Nine Mask Awakening which grants the user nine masked beasts summoning.
Not exactly sure how masked beast controls would work. It could be a mixture of summoning heart demons and using six paths. You can either manipulate the masked beast like a puppet or use it as a body.
-Seiryu
A dragon that resembles a sea horse. Ninjutsu Based. Comes with a flamethrower.
-Byakko
A white tiger with black stripes. Taijutsu Based. Comes with Five Claws Slash/ Tiger Slash. Slashes the any target within a 3 tile rangeof the user with a 5 tile wide width.
-Suzaku
A large bird. Taijutsu Based. Comes with Whirlwind and Razor Beak. Whirlwhind similar to konan's tornado. Razor beak has a 1 tile wide and 3 range.
-Genbu
A tottoise shell. Taijutsu Based. Comes with Bind. Bind has 1 tile width with a 2 tile range. The masked beast binds the target with its body and does damage. Similar to the puppet technique.
Kinja
A large snake with golden scales. Ninjutsu based. Comes with snake bind and a poison technique.
Tennyo
An angelic priestest with with long flowing robes. Taijutsu based. Comes with Robe Whip and Robe Bind.
-Shinigami
Death God, Resembles a skeleton and carries a scythe. Ninjutsu Based. Comes with Reaper Death Seal and Chakra Wave. Chakra Wave does not do damage but drains chakra.
Hokuto Sennin & Nanto Sennin (Shhould be summoned together)
They are both priest looking with 1 staff each. Ninjutsu Based. Comes with Chakra Barrier and chakra Bind. Chakra barrier is 4 tiles wide and blocks only ninjutsu projectiles. Chakra Bind forms a purple cylinder around the target trapping them for some seconds.
More could be added to this if its used but this is the general idea for now.