I'd bring over Nioh's character creation tools to Ninja Gaiden 4.
Just make DOA Venus Vacation a dating Sim

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I wouldn't mind a FF7R style remake for the SNES Ninja Gaiden games. I'd replace Ryu with his father Jo. The SNES games were his adventures, the XBOX games were Ryu's.
There are some continuity issues but you could fix them by adapting them out. Ryu's father was named either Ken or Jo depending on the game. So make Ken Jo's father and kill him off.
You could always scrap the whole "Ryu marries Irene and retires" aspect.
Replacing Ryu isn't necessary but using Jo opens more opportunities.
DOA characters should make a guest appearance.