Older people. We live in a rapidly ageing society. Retirees are a large and wealthy demographic. Despite that, tech companies are absolutely woeful at designing products for older users. We don't empathise with their needs. We don't understand how poor eyesight, arthritis or cognitive difficulties can affect UX. There's a huge amount of pent-up demand and excellent opportunities for future growth.
There are certain near-future technologies like self-driving cars and robotic home care that could revolutionise the lives of the old. I think that's a given, but they have a relatively high barrier to entry.
At the other end of the scale there must be pure (or almost pure) software solutions that could help older peoples' lives also.
I've been thinking of Alexa-style AIs that actually perform useful tasks such as pre-screening scam phone calls, alerting the user about important emails, or proactively helping the user perform a bank transaction. This could be useful for everyone, but when an old person can barely read emails, never mind sort important ones from the scams and junk, I think it would be a godsend.
The devil is of course in the detail. As a high-level concept it all sounds great, but how would it actually work? I've been imagining a kind of "meta-OS" that sits on top of the OS and drives it on the user's behalf. Or perhaps it should be more like a netbook, with all the actual content kept in the cloud, and the AI as a thin client dedicated solely to allowing easy access to the data. There are a lot of possible way for this to work, and a huge number of problems to solve...