The suggestion I've received from other people I've talked to, is to learn Unity, make it myself (or with help from people more competent than myself) and submit it to a publisher. I'm not sure how feasible this is, ...

...to be perfectly honest, I have little expectation of this stuff actually going anywhere. I'm not a game designer, I have no history in the industry...

Clearly, you should not do that; the chances of success are low in the extreme. If you're not in the industry and don't plan to make a career in the industry, just put the idea aside. If you plan to make a career in the industry, put the idea aside for now, and come back to it when you're ready and have the knowledge and contacts and resources.

The only way you are really going to see your "idea" come to fruition is either learn to make the game yourself or find the cash to be able to hire a team / found a company with the skill set to do it for / with you.

On the plus side:

My concept is for a Warhammer 40,000 stealth/third person shooter

Games Workshop seem to be giving out licences for their IP as though they were cereal box toys these days.

Sounds like you have a passion for Warhammer and the stories it inspires.

In this situation, you need to focus on what is possible - not impossible. Using Warhammer name, characters, events, designs etc - thats impossible from a practical point of view. Producing a full-blown AAA 3D title is also impossible from a practical point of view. However, what IS possible is taking your basic idea of "consequences" and development of the player character, and the party that follows them and applying that to a simplified 2D game with a "universe" of your design.

What is also worth considering - and strongly advised - is taking a look at how a games series develop with each game. You might not be able to apply all of your ideas in the early entries but thats fine as it leaves a challenge for the games to come...

Look at the games of id software. Doom took the world by storm, but it was only possible by id making Wolfenstein3D first. And Wolf3D itself was only possible with the Catacomb games and even then, further back, a game about a tank rescuing civilians from monsters(I think? Pretty sure it was)...

To get around the issue of inspiration of Warhammer - look at the development of the game itself. Its just the classic game of historically-based war games - played by real-life generals - given a sci-fi setting. Hell, most of Warhammer 40K was a rip-off of ALIENS, Starship Troopers etc and thrown in with the usual Tolkien-stuff like Orcs, Elves, Undead etc...

If you have little interest in programming, but willing to put in a bit of hard-graft, then maybe look at either Unity or Gamemaker to at least knock together a "taster". It will take a lot of hard graft, but with such packages - especially Gamemaker - you could knock out a 2D version of your vision. Set aside the next two or three years and you'll probably accomplish that goal. Just, move away from Warhammer, but keep that warm feeling it gives you.

Anyway, good luck!

but the idea is bugging the hell out of me.

Maybe you just need to make some fanfiction ?
Rules about IPs are generally not enforced/sued for when making fan-fiction,

and you might find some warhammer-fans to help you.

It wouldn't be a triple A title, but then again, who still plays those nowadays ?

My concept is for a Warhammer 40,000 stealth/third person shooter with heavy RPG elements. The player character would be an Interrogator for the Inquisition. You would have a variety of options for what sort of background you would have, tech priest, psyker, guardsman, ex-mercenary, that sort of thing.I personally like the idea of being able to play through your PC's rise from acolyte to Interrogator, but I recognize this that is most likely impractical. You would have a variety of characters from your Inquisitor's retinue to use, all of them would of course have their own backgrounds, stories etc. But in keeping with the tone of 40k, every one of them would be able to die during the game. Not as a result of a bullshit no win situations, always as a result of decisions you make. Anytime someone dies, it is absolutely the player's fault and it can be avoided. Other things that can happen as a result of bad decisions should be things like plagues, massive victories by xeno or heretical forces, and the deaths of many guardsmen. Detective work should be a major component, shooting your way through should be possible, but not ideal.

Well, apply for a job at GamesWorkshop (or any 3rd party they deal with) as a junior game designer and work your ass off up 'till you get the Game Director's position (in some way)