The invention I really wish existed is a small device that could instantly translate sign language into speech, and speech back into text, in real time. I'm a software tester in Hanoi, so I think about how technology could work in daily life quite a lot.
I imagine it as a pair of lightweight glasses with tiny cameras. When a deaf person signs, the cameras would read their hand movements and facial expressions, and a little speaker would say the words out loud. Then when someone replies, the words would appear as subtitles inside the lenses, so the deaf person could read them straight away.
Obviously the main users would be people who are deaf or hard of hearing, but also their families, teachers, doctors and shop assistants, basically anyone who needs to communicate with them but doesn't know sign language.
The reason I'd love this to exist is a bit personal. My cousin Linh was born deaf, and when we were kids I always felt guilty that I only learned a handful of signs. At family gatherings she'd often sit quietly while everyone else was chatting, and I could tell she felt left out. Now she's grown up and works as a graphic designer, but she still finds job interviews and doctor's appointments really stressful because she has to rely on someone else to interpret. A device like this would give her independence and privacy. I know there are apps that do bits of this, but nothing is accurate or quick enough yet. If someone cracked it, I think it would change millions of lives overnight.