>It could pop up upon pressing another button so it doesn't 'clutter' the interface for everyone who browses /lit/

A popup won't work, we need to support users without JavaScript, remember. I suppose adding a single input field above the post message in the form won't make it any more cluttered, and for viewers the entire form is hidden by default anyway.

>Do you envision a sort by or search by tag function,how would you implement it?

Sure, the ultimate goal is search by tags as on booru-like boards.