I’m trying to think of the last game-changing site, my guess is ManyVids? Not sure when they opened. But they are the most recent site I can think of that had much of an impact, and they’re a content site, not a cam site. They filled a nice, surprisingly basic niche (somewhere for vanilla models to sell videos, and for guys to go for basic, non-fetish/dom stuff), and partnered with some really successful models at a time when most weren’t selling on non-MFC sites.
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Manyvids also hit a really good one when they allowed you to host your videos on their site and send a free download link to members. This meant at the time that whenever a lot of girls were on MFC they had a link to their Manyvids store and every time they sold a video they'd be linking someone over to Manyvids where they'd have to make a free account to download the video. Now of course MFC has tapped into the video market (finally) and this is no longer allowed as videos can be purchased directly through MFC share.
Honestly as everyone else has said, in terms of a regular run of the mill camsite you're unlikely to grab a new niche. You could maybe do a kink only site (I don't know if there are any already), which could attract kink based models and members. Clips4Sale has tapped into this market in terms of clips, but I don't know if you can do live camming there. I also don't know how well live kink stuff would pay as much of it needs to be prepared in advance. The OP actually comes across very nice and professional which is rare, so I'll give my best tips.
Another option could be tapping into the online gaming market and creating a Twitch competitor which allows streamers more freedom in terms of outfits. I don't know how this would work in terms of legality due to potential underage viewers, but even if there weren't nudity, having a more encouraged tip culture and allowing performers to do more stuff like cosplay without kicking them off would be cool. You might also be able to bring in a fair few former Twitch streamers/youtubers who were kicked off Twitch. There is clearly a market for watching hot girls gaming in cute outfits/underwear, how big I do not know, but an adult's only version of Twitch might potentially work. Bear in mind that MFC has already being creating a non nude spin off of MFC which has been in beta for about a year now. Like any site it may well completely flop.
You're not in England so this probably wouldn't be possible, but there is a site here called AdultWork which as well as being a cam site also allows independent escorts to advertise and book from their site. Prostitution is legal in the UK, though I am not entirely sure about how the legality of this works as advertising services is not technically legal. The site Adultwork is notorious for being badly run and not treating those using the site for work well. I imagine if someone with a professional, sex worker positive attitude wanted to create a website offering similar services, it may not be hard to get the workers to cross over. But you would have to be based in the UK I think so that's probably not necessarily the best idea. I have considered this as a project as it's clearly a requirement, but I'm not big on creating websites so I'd have to employ others to do all the work. I mention this here as there are lots of escorts being treated poorly by this horrible site yet not having anywhere else to work from, so if someone were to have a look into that idea it might be worthwhile. I put emphasis on being sex worker positive though. The last thing anyone needs is another arsehole who thinks sex workers are scum getting involved in the industry.
Honestly, as others have already mentioned, this is currently not a great industry to get into. The big sites are now huge, and are catching onto what models want/don't want already. Some sites I didn't ever believe would go anywhere are still running, so it is possible to get in there, but at this point the market is saturated. You're really going to have to spend a long time researching to find the niche. I have no idea if there truly is one in mainstream camsites.
The only other niche I can think of is a really good camsite aimed towards straight women, I thought of this years ago and still have no idea if it'd make money. It might though. It'd have to be very different to camsites aimed towards men though, and even sites which allow male models to cam. Probably more like MFC/Twitch in terms of friendship/conversation rather than just guys who want to get their junk out on display. The quality of the models would need to be good in terms of being approachable and good to chat to/entertaining to watch without being intimidating. I have no idea if this type of site would make a penny, though with more women working hard and having good income, and many divorced women with spare cash there might be a niche. Women are also gradually becoming more looks focused. I guess it'd be like "boyfriend experience". Might be worth doing some research into.