I've wanted to build something like my idea for years, but it's not something you can do on the cheap. I've thought about focusing on some vertical to make it cheaper, but meh.
Eg) I have one for /objc that searches a half-dozen high-quality, good signal-to-noise, non-spammy objective-c related sites, so that if I search for "timers /objc" I get only quality content and no farmed spam.
There's a lot more to it but they've essentially farmed out the job of whitelisting the non-spammy parts of the internet to their users.
Edit: and more to the point of what you were desribing, you can easily use other people's public slashtags, and it will detect and suggest relevant ones as you use it. It's totally worth playing around with.