You can't have scary and heavy metal at the same time.
When will people understand, Doom is a game of powerful constrasts, no vision of the game in true or fales, they are all true to a certain degree.
One moment you are in a dark, eerie and cramped room with monster closets, the next you are in a bright, open area. One moment you hear heavy metal midis, the next creepy and mysterious tracks, one moment you are in an Alienesque base, the next you are in the downright weird Doom 2, one moment you fight hordes of monsters, the next just a few imps.
Doom is HARDLY just heavy metal and 30 monsters on the screen. Of course, it is a very important part of the game but there are other parts of it too.
As for gameplay, even Doom has to evolve, but in a way that makes sense for the franchise, I've already laid out my ideas for themed episodes that introduce weapons based on those themes, with gameplay twists on the Doom formula every episode.
People keep talking about Painkiller and Serious Sam as "perfect" references for a modern Doom sequel, but the fact is that Doom has far more in common with 2d sidescrollers only in 3d.
Secret hunting, searching for keys, backtracking, emphasis on maneuverability are all taken from 2d sidescrollers such as Commander Keen and even Mario but in 3D and with demons from hell.
Other things they could do is add vehicles. Now I know what you're thinking but you don't hate vehicles just because they are vehicles, you hate them because vehicles are often just glorified turret sections.
One of the very best vehicles in gaming is, I believe Yoshi. Why?
Well let's see.
You are NOT railroaded into riding him, the levels are perfectly navigable without him.
He has interesting gameplay mechanics such as swallowing enemies, which can be thrown back, after he eats enough, he lays an egg with a random powerup.
Moreover, he also gives you extra possibilities of exploring the levels with his light flying abilities.
In short, the mechanics in sidescrollers like Mario are far more complex that what you see in your average MMS, every time a modern game makes something similar to Yoshi, it is considered revolutionary, when mario already did it before.
Now imagine that instead of a cute dinosaur you get a vehicle with light hovering capabilities that provides extra ways to find secrets, has a powerful cannon that destroys demons but has limited ammo, and you have to manage fuel/ammo. The vehicle is simply just another way of advancing/exploring the level, the game does not railroad you into using it.
I believe that after a certain amount of time, the more you have a weapon/vehicle the less interesting it becomes. Special weapons/vehicles should be made in a way that they are rare, powerful, offer new strategies and the resources have to be managed carefully because they deplete rapidly.
Here is another example. You are in a UAC space station that has a ship port at one point of the level. Here you can find a remaining smaller ship that can be manned. This provides the player two possibilities, either he ignores the ship and makes his way through the starship OR he uses the ship to get there faster. The catch is that the ship has little ammo left and the player is assaulted by lots of cacodemons from space. Even if you take the ship, you can still return at any time to the port in case you run out of ammo or the cacodemons prove to be too strong.
See? Instead of a glorified turret section, the vehicle now provides the player with an extra route, which also adds replayability.