It doesn't need to be extraordinary. It needs mass appeal. I believe McDonalds is the largest restaurant chain in the world.
It also has to be affordable to the masses, of high quality, AND profitable.
Unfortunately in todays market that means it'll be made in China and that opens the door to many quality issues. The added cost of shipping an importing large numbers these days may make the affordability part a problem if its to remain profitable.
The area you market it in is also a big factor.
I'm far from the EU and things around here when it comes to hifi are bleak. There's not been a successful store here in 25 years or more. Most who I know personally that are into hifi to any degree are in it only if its used and cheap. I likely fall into that category. I've got a house full of really nice gear but its a collection that I accumulated over over many years. As long as there are estate sales, flea markets, and even CL or FB the chances of me ever looking into anything new, unless its dirt cheap are pretty slim.
I've got one buddy who's got five times as much a I do, 45 years ago he spend stupid money on stereo gear buying only the best of the best from the big player brands of that time. He's bought many items since but his budget in his late 60's is limited to what he finds at auctions and yard sales. His budget tends to be around $25 or so.
As ridiculous as that may sound, there's no shortage of folks out there with gear they no longer want or have no idea what its really worth. If there weren't I wouldn't be looking at unloading two pair of JBL XPL200A's when the sun comes up that I bought through a CL ad for a pair of speaker stands for free. I spent more in fuel getting there then I did for the speakers, and a slew of other gear I've not even yet looked at. The seller felt the stands looked like something someone might be able to use so she listed them, but through the speakers were too big to be wanted by anyone so she didn't bother and said she was just waiting for her cousin to stop by to help her get them out of the house. Stuff like that is why I keep a folding handtruck in my truck. I had to argue with her to take a pair of $20 bills, which was all I had with me. I sort of got the impression it may have been a case of a widow selling things for what her husband told her he paid but i didn't ask too many questions.
I didn't need to spend $40, it'll come out of my food budget for the month I suppose but I couldn't leave them. I don't suppose anyone here would.
Just last week i swore I wasn't going to bring anything else home until I got rid of some stuff, but i was wrong.
This makes four pair of these now. Every one was free. Most of my best speakers, all larger, were free.
I think the most i ever forked over for anything was a $100 for stack of components that included two Yamaha M65 amps a Technics SL1300 TT, an Harman Kardon Preamp, and a Nakamichi cassette deck. And that wasn't even all cash, in that I traded a Sony Mini system and $20 for it that I had replace the belts on.
I'm sure there's a ton of great gear out there for sale new, but I'm totally content to wait till its free or close to it to try it.
If I never found another piece of hifi, I'd still not likely live long enough to go through and try everything I've got now, and if deals keep falling in my lap, I suppose not much will change in the years to come.
Of course as I get older, it means I can go 'shopping' in my own attic and find new things I completely forgot i owned over and over again. I've probably lost more gear around the house and in the attic than I've bought in the last few years. Just yesterday I stumbled on two pair of Boston T-1030 speakers I didn't realize I had and two Pioneer SX-1050 receivers. If I hadn't bought 12 new shelves to match what's already up there, I wouldn't have been emptying and moving all the rows of shelves to be able to fit in another one to find room for what I bought a month ago. My best guess is that back when I bought my first pair, that I really liked, I most likely bought a few others that popped up as well so I'd have a few spare sets. The only problem is I likely did that several times because it means I've got 6 pair of BA T-1000/1030 speakers. Not to mention two pair of each of all the other models as well.
In someways its hoarding, but in other ways its shopping for the future. In comparison, I've got several buddies who's collection makes what I have here look amateur. Since I paid next to nothing, or nothing for almost all of it, it also means there's no market for it. If there was, it wouldn't have been free. I rarely answer an ad when it first pops up, it usually takes me staring at it for a month and wondering why its still up to actually respond. Then if it sounds interesting and cheap enough I'll think about taking the drive to go look. I rarely just "look" though not wanting to come home having wasted the fuel to go there and back.