On Tumblr in Xkit Rewritten, you're able to create tag bundles. You can title each tag bundle, and put however many tags within it. When you click on it, you can just tag the post with all of those tags.
Something like that would be super useful for me here, especially since I'm moving a lot of content from Twitter and Tumblr to here.
Also, if people spamming tags is a concern - Maybe make it a feature that you can't access until after a week of your account being activated or something?
I was just about to suggest this but the title search thing pointed me to this existing issue instead, I think this would be a great idea!