We’ve had internet sh*tfights over this. As a service which has federated with other providers - including commercial providers, some services get very upset if you “steal” their hashtags and point them at your own site. And we have every right to get upset if somebody “steals” them from us.
So point them at your own site and don’t rewrite any hashtag links that point to another site. If somebody comes up with a global fediverse hashtag search - feel free to offer it as a service in addition to not stealing hashtags. I probably will not use a global fediverse hashtag search because it doesn’t scale; and if it does it needs monetisation and therein lies a slippery slope back to centralised services; because that’s basically what it is.
A number of projects (including mine) offer the ability to “follow hashtags”. You only follow the hashtags which already arrive at your site. Many have suggested ways of subscribing to hashtags from other sites, but it’s basically a poor man’s groups. Just use groups. Because as soon as these hashtags become popular, they will get hijacked for spam. At least a group has an owner that can kick people out. It’s a lot harder to kick people out of a hashtag.
Diaspora built their network on shared hashtags - it was the only practical discovery mechanism they had; and it led to centralisation, because following hashtags provides fewer results on smaller sites and it tends to drive people to the largest sites. So it’s the slippery slope problem. Again we offer them as a service, so you might think I’m being a hypocrite; but we acknowledge their limits and their tendency to push people to large sites. It’s just another tool in our chest full of useful tools. We also have groups. We’ve found most people prefer groups and when both are available, they don’t rely on hashtag following (which would drive them to large sites), but just use it as another tool and aren’t necessarily disappointed if they’re on a small site and don’t get a lot of followed hashtag content. If followed hashtags are all they’ve got, they get hopping mad that they can’t see every matching hashtag in the network and start looking for centralised solutions to this perceived problem.
Sorry for rambling, but it’s a diverse topic with a lot of implications.