Not sure if this already exists as an external app or if there's anything that's close to this, but I think the site would be greatly improved if it had a functionality that recommends posts based on the tags (or other metrics) of what you favorite and upvote.
My reasoning is E621 has the best tagging system and most content that doesn't include very low quality or duplicates, but one thing it's lacking is that due to it having so much content it often comes to browsing hundreds or thousands of posts to only end up finding a couple ones that fit your tastes, and sometimes it gets a bit too long to check new posts daily (even worse if you take a break), so adding a feed or algorithm similar to Twitter or other sites, or something like Pinterest might really eliminate the issue of having to look at pages of content where only a one to two percent of it really interests you.
Like I said, I don't know if this already exist or if there's something that's just the next best thing that eliminates the purpose of having a whole new system be made. I also don't know how much work this would be, devs might already be busy enough as it is or there isn't the money or the time.
Also not sure if this is supposed to go in another category, I'm not really expecting this to go to the point of it actually being added so I'm more seeking to discuss about it, hence the category choice.