Something that would feasibly go into production is hard to narrow down, as most gadgets are invasive or dangerous, such as laser, exploding or buzz saw watches, credit card lock picks or safe cracking devices. I think Bond's glasses in AVTAK might be the most practical and innocent - as dated as they are. To see through tinted/mirrored windows might be popular.
But I'd love to own Bond's phone in TND or a working laser watch.
I'm moving this to the General Discussion section because it's not so much related to Spectre specifically.