I must say, if you want to build something that instantly has a market consider building plugins for existing 3rd party systems that allow you to charge a fee. It ends up being a much smaller project than trying to build your own, huge web app, and it comes with a targeted, focused audience who can easily install your app with a couple clicks (and get charged). You don't really have to market anything.
I started looking for opportunities in WordPress a couple years ago. Anytime I found a new app, I got in the habit of checking if it existed as a WordPress plugin. I've since built a nice side business that I hope to grow into a day job.
Semi-tangent: about 2 months ago someone posted a site that listed potential SaaS eligible services requested and suggested price points, does anyone recall that url?
Find a domain name with matching social media handles. Instantly searches as-you-type. Also provides some simple "suggestions" based on common startup prefixes and suffixes.
Monetized via affiliate commissions on referred domain sales.
Selling Rsync/ZFS-Storage as a service at $.05 per GB - figured how to combine Cygwin Rsync and Volume Shadow Service on windows to backup locked files.
nothing to contribute on my own, but this may interest you: https://www.indiehackers.com/businesses - probably something worth reading as it goes in to ideas and their implementation. interesting to note that there is a weather app chrome extension there that seems to make 2.5k per month.
Thrilled this exists and monetization was figured out. Years ago, I used a similar service that shut down unexpectedly because the creator couldn't figure out how to monetize it. I've thought about rebuilding it myself twice. I even own the domains! :-)
I wouldn't call it a business per-se. It seems about once every month or two I can organize a connection, but I have thought about making it a bit more commercial.
GPU's aren't used for mining Bitcoin. Bitcoin's algorithm is CPU-intensive, and is nearly exclusively done on from-the-Silicon-up purpose-built chips like Avalon and Antminer.
GPU's are used for mining other cryptocurrencies like Litecoin and Ethereum. These can be traded for Bitcoin or fiat currencies.
Bitcoin was GPU mined for a while (it's "just" SHA-256, it's not particularly designed to be GPU-hard unlike other PoW algorithms) but you're right that since ASIC mining became the norm it's not really cost-effective to mine BTC on the GPU.
Some other cryptocurrencies have algorithms that are designed to be hard for GPUs and ASICs (cryptonote for instance) but I think it's just postponing the inevitable. If those altcoins become valuable enough I'm sure people will start making micro-optimized dedicated hardware to mine them and eventually it won't make sense to use GPUs for them either.
I must say, if you want to build something that instantly has a market consider building plugins for existing 3rd party systems that allow you to charge a fee. It ends up being a much smaller project than trying to build your own, huge web app, and it comes with a targeted, focused audience who can easily install your app with a couple clicks (and get charged). You don't really have to market anything.
Build something good and people will find it.