My idea would be pretty simple, yet could be made grade A awesome if done right.
It wouldn't be as deep as DMC5 but would be pretty much on par with the good God of War games(the ones that aren't GoW2018).
(Story & Character)
For the character, I'd prefer a female character to be a "Bad Girl" archetype that could look alien as well as stylish and iconic as well as to spit in the face of the so called "realistic" designs of today.
Something like the designs of Webwitch or Witchblade:
The Story would be pretty basic:
A symbiotic alien species invades and attacks earth, protag "absorbs" the alien power and uses it to fight back against the aliens.
The game's levels could take us from alien infested cities, to alien space ships, even to other planets and the like.
The actual story could involve something similar to "The Darkness" were the alien infection battles with the protag's conscious for control.
(Gameplay)
The gameplay would be similar to that of the good God of War games and even Bayonetta where the combat is brutal, violent and the main character's main attacks are tentacles that work similar to Krato's Blades of Chaos or Bayo's Alruna and she'd also have claw attacks as well.
Combat would also feature quick finishing moves similar to Doom's glory kills.
I'd love for a game like this to show up, especially in this day and age.