Depending on your ethics, blockchain/crypto/DeFi (or ReFi, whatever that means, just saw the term today) and "web3" could be an option. It's not sustainable as ultimately it's either solving a problem that doesn't exist or poorly solving a problem that can be solved much more efficiently without crypto/blockchains, but there's a lot of capital and clueless investors/idiots going around that'll be happy to throw money your way if you can drum up some marketing & PR.
In terms of actual, sustainable businesses, looking beyond tech is an option. Outside of tech there's a lot of industries that still operate very inefficiently and a modern software solution combined with IoT could help - here's an example: https://news.ycombinator.com/item?id=26240581 and I'm sure there are plenty more. I'm watching the YT channel of an HVAC technician and he's still working on a lot of equipment that's 10-15 years old and clients are cheap and want to run it into the ground - no preventative maintenance and as a result a lot of his calls are "reactive maintenance" aka emergency calls when the equipment is already down. Seems like a low-cost box with a few sensors and IO lines can easily be retrofitted and would allow remote monitoring & proactive failure detection (a vibration sensor alone would probably help) and if cheap enough could be interesting to clients (or even the technician himself, if it helps him get more business or better schedule jobs around predicted failures rather than everything being an emergency job).
In terms of actual, sustainable businesses, looking beyond tech is an option. Outside of tech there's a lot of industries that still operate very inefficiently and a modern software solution combined with IoT could help - here's an example: https://news.ycombinator.com/item?id=26240581 and I'm sure there are plenty more. I'm watching the YT channel of an HVAC technician and he's still working on a lot of equipment that's 10-15 years old and clients are cheap and want to run it into the ground - no preventative maintenance and as a result a lot of his calls are "reactive maintenance" aka emergency calls when the equipment is already down. Seems like a low-cost box with a few sensors and IO lines can easily be retrofitted and would allow remote monitoring & proactive failure detection (a vibration sensor alone would probably help) and if cheap enough could be interesting to clients (or even the technician himself, if it helps him get more business or better schedule jobs around predicted failures rather than everything being an emergency job).