I'm afraid I don't understand exactly what you mean by 'tags' there! Do you mean icon keywords? There's a bug open for merging two icon keywords into one (and I'm not 100% positive but I think there's a Horrible Hack you can use to do it even before then -- maybe somebody reading can let me know if I'm misremembering) -- nobody's been able to get to it yet, but it is in the bug tracker and somebody will get to it sooner or later.

I think that doing a list of all the keywords you've used in the past would be computationally expensive, but I'm not 100% positive; that might be possible. That sounds like an excellent candidate for a suggestion!