As for Thunderbird itself, I have only two wishes:
1. Support Exchange calendaring natively and reliably. I've used different extensions (like Lightning along with DavMail or Exchange Provider), and have always had to revert back to OWA (Outlook Web Access) just to have a usable calendar. Without this, I cannot even recommend Thunderbird within my organization (where almost everybody else is stuck with Outlook; this is not the only factor that makes Outlook attractive to users in the recent years — Outlook also has good integration with Skype and Skype Meetings in the calendar, where people start chats on Skype based on calendar events).
2. Make it perform better. Most people who are Thunderbird users surely have been using it for several years and use multiple mailboxes, with each being at least a few GBs in size. Very few users adopt regular archival, folders, inbox zero and other approaches. I have multiple mailboxes, with a couple of them where the Inbox is about 1GB or 2GB. Thunderbird takes several minutes to startup and show its screen. I know it uses MSF files for the indexes, but it doesn't seem like it does an optimized load of the mail data (mbox) file or takes a lazy approach to loading information that may not be recent. Downloading mail content from IMAP also seems to be slow, though that could be a combination of the servers (across providers) being slower, the protocol and Thunderbird's design/implementation.
P.S.: I donate regularly to the Thunderbird project to show my support. Here's the link for the English version of the donation page (the donation goes only to the Thunderbird project and not to Mozilla/other projects) — https://donate.mozilla.org/en-US/thunderbird/