I'd do a hard reboot of Resident Evil.
Ditch all the convoluted continuity and 'characters' and everything.
In designing the game I'd focus on the title (Resident Evil) and make the theme actually relevant to it, rather than the game being Biohazard and just being called RE for the states version. So, it would probably deal with themes of inherent violence and evil in both nature , and people.
Okay, I'd also change up the representation of the titular 'evil' from biohazards and evil corporations, to unexplainable supernatural-like phenomenon and cult activity. Even though this is a big shift for the series, I would definitely keep the core spirit of RE alive and have plenty of homages (that I will get to later).
Tonally the game needs to be scary as hell, and while it would be focused primarily on body horror and tense enemy management, there would also be some disturbing psychological horror. The main character (either a new character, or a rebooted version of Leon) could be kind of a smartass so there could be a little bit of humor, but overall I'd want the game to be serious and have some actual dramatic heft in the writing.
The game would have you travel to a remote mountain town in the United States where you are investigating child disappearances (the reason you can't leave or call in back up with be explained with some cult magic, like when you first arrive you see the town is surrounded by a ring of salt and later you find out that was part of some ritual or something). The scares would ramp up a little slower as members of a mysterious cult led by a prodigal son of a wealthy mansion owner show up, and they start performing rituals on the said disappearing children and bringing creatures into the world as well as changing the townspeople into ganado like freaks. And then you get to explore the town, forest, swamp, fields, and cliffs that make up an open world. I wouldn't want a huge GTA style open world, but rather something that is densely crafted to perfection, and feels manageable to explore but with plenty of secrets and optional scary 'sidequests' you can find.
At the center of the map however there is a big old mansion that a decent chunk of the story is set in (maybe 30%) but that won't unlock all of it. So you can explore the mansion and unlock rooms in between story sections if you want, and find secrets (like pages of some cult book that act as a cool illustrated bestiary), as well as some spooky shit once you unlock the basement or attic. This would basically be a throw-back to RE1. There would also be access to cars and other vehicles to help you travel, but those can get mobbed by enemies (imagine driving in a lightning storm, only to crash into a tree as a mob of freaky villagers hunt you down).
As you explore the world if you run into enemies you can either avoid them via stealth or take them on with your limited resources (you really have to decide what would be best, and on harder difficulty you would have less resources so you would have to rely more on stealth). Environmental progression would mimic the old RE games in that you slowly 'unlock' more areas of the map, and there will be frequent returns to areas you have already been to but there will be a new twist or scare when you go back.
As for boss fights I'd take a little inspiration from MGS in that there would be quite a bit of them, and a ton of effort would go into them. Like one boss fight could be set inside a house you are locked in against a chainsaw wielding maniac, and another could be against some creepy serpentine corpse-monster in the swamp where it can pull you underwater. And even a boss that can hunt you down throughout the whole world map.
As for controls it would take a little inspiration from The Last of Us but even more fluid, because I'd want combat and stealth to be super dynamic, and chases to be fast paced, fluid, and terrifying. An example of why fluid controls in RE could be awesoem: you're being chased through a little neighborhood area, you just manage to hop a fence to lose your pursuers and you stealth around, only to be jumped by and enemy who you wrestle off you and the horde is on you again so you open fire behind you as you run to the nearest car, or house to blockade yourself into.
There would also be some form of inventory box, and weapons collection (although I'd want the weapons to be more grounded). Also you would be able to pick up melee weapons from the environment (think condemned) because you will be frequently out of ammo.
