Some of these features include
- option to select date range(probably by month)
- new format printer-friendly PDF with bookmarks, TOC;
- tweetbook for search results
- tweetbook for tweets from a user's home timeline.
While the pure pleasure of designing & coding and being appreciated is something to be cherished, the site doesn't make enough to pay for its hosting charges. Advertisements in PDF, providing premium service and affiliating with publishing companies are few options available (in the order of decreasing ease of implementation) for sustaining the site.
Though not inclined towards the idea of having ads in PDF, proper non-intrusive placement of 'right' ads might be acceptable. A premium service which adds extra bells & whistles to tweetbook would be nice but being a one-man-self-critical team, it would take some time. Affiliating with publishing companies is something that has to be explored.
I would appreciate your thoughts and opinions in the comments section.