“Formalising” my suggestions from the Discord server:
Tag incompatibilities. For example, “gradient background” says that by definition it cannot also be a “simple background”, so adding ‘gradient’ would block/remove ‘simple’. This would be applied to any tags that are often misused or contradict one another, like rating tags, adding solo and duo, absurd res and low res, using ‘safe’ and any explicit tag, etc.
For the “derpibooru exclusive” tag to change the source input to “Derpibooru exclusive”. It has always bothered me that exclusive art is permanently marked with “not provided yet”.
And adding a +1 for tag markup for text (ie. >>pony or [[pony]]), instead of having to manually add links. If you’re fancy, you could even have it show the graphic tag and its number. This mainly benefits mods and tagging submissions, because when you’re adding 10+ tags to a post, the time to search and copy-paste addresses really adds up (and manually typing can lead to incorrect unicode).