I see huge potential in Thunderbird. I have myself been a long time Mozilla supporter and donator. I feel that Thunderbird has a huge potential to create a seamless ecosystem people are looking for. I am confident that thunderbird can be the best email client and suite, and show that the open source projects can beat private proprietary projects.
I have some points below that I think can be an amazing addition to the Mozilla Thunderbird, now for some there may be 3rd party extensions, I have found them to be not very stable, and I don't think every user is tech-savvy and aware enough to find extensions and go through the whole hassle, whereas in other application that is by default there. If Thunderbird has to become the best, I think it will need to have some of these at least.
Except basic features, I do not expect all of it to be free, Thunderbird can charge a small amount for users who want to use these premium features and sustain its development and future growth, It's sad to see organizations like this have to let people go.
1. Has Thunderbird grow as such that it can replace Microsoft Office Suite. Integrated LibreOffice for edit inside Thunderbird.
2. Firefox Send, integrated inside the Thunderbird and automatically uploads and sends the file using the Firefox send. To avoid spamming, Firefox account login can be used.
3. Phishing reporting button, a button like outlook that can notify if it's a phishing email and if user feels it is then can click on the button and submit that email as phishing email. - this feature can be used by the enterprises who plan to adopt Thunderbird.
4. Firefox account login has grown a lot over time, and I see that Thunderbird is also getting Firefox sync. If Firefox account can be used as a one-stop shop for everything, then it can be an amazing ecosystem builder. Firefox's account can be used to sync calendar.
5. Ability to create meetings and invite users from thunderbird calendar, if Thunderbird can have its own integrated application for calls and messages to other Thunderbird users using Firefox account, that can be an awesome feature. There are various open source applications available like Signal, its protocol and features can be used inside thunderbird.
6. Ability to run, share, and collaborate LibreOffice inside thunderbird. I think this can be a huge step forward in the direction of a robust office suite. Features like encryption and Firefox send can help massive collaboration in teams and small enterprise.
7. VPN support in Thunderbird - Very important for businesses and enterprises.
8. Clean and beautiful calendar - Right now the calendar is not.
9. A better address book management - New email addresses should be also synced with Firefox account, if it can be provided as an option to sync with the OS's address book that can be a good optional feature.
10. 3rd Party sign in for sync - Important for enterprise and business users.
11. Mozilla has invested in multiple startups and projects, heylogin is one of them. Heylogin integration in Thunderbird can be amazing. Can be very beneficial for all users and for sure for businesses.
12. A better task management section - there are so many reminders and to-do list applications available and are much beautiful. A better task manager is required with clean UI and collaboration feature. With project tracker and UI options for project planning and tracking. There are multiple apps for this like Todoist, Notion, Trello, Jira and many more. Some ideas can be derived from this and applied in Thunderbird. Can be amazing for personal and business users.
13. Dark mode option for email and UI inbuilt in client.
14. Cleaner UI - Email reading space in vertical visibility is very less, so much screen is gone with too many details and buttons which can be shrunk and made much cleaner.
15. Email conversation - A better and cleaner Email conversation chain representation. Conversation to be ascending and the list of emails in a folder descending. The visibility of conversation chain is confusing and not much user-friendly.