Be close to the existing Thunderbird
Even though we write almost all code from scratch, we will save a lot of time by having a clear goal: We want to replicate the current Thunderbird, from an end user perspective. That means, the user will find the same 3-pane window layout, the same way how folders and message lists and the thread pane operate. The theme will be similar. Existing Thunderbird users should feel right at home.
We retain the overall UI and most features and qualities like performance, even if we do not copy all little details.
Disclaimer: Given that the technological basis - particularly HTML - is completely different, there will be some things that work differently in some ways. Hopefully, many will be better. We will have some technical limitations. Some will be just different, because the underlying implementation is completely different. The goal is not to copy bug for bug, but to make existing users immediately recognize this as Thunderbird, and feel at home, even if some details are different.
We must pay attention to also keep technical qualities that many of our users rely on. An obvious one is that the new implementation must be able to quickly scroll through a list of up to 100000 messages. There is no such HTML widget that allows that, we would have to create one, but I think it's feasible, I already have ideas how to do that. We also need to preserve privacy and security qualities that Thunderbird has, or even improve on it.
Fresh UI, more platforms
In addition to replicating the current Thunderbird UI, we should also experiment with new forms of UI, in parallel. For example, we should create a UI that's suitable for the new generation of users that never used a desktop email client before. These people do not feel at home with Thunderbird today, and we should create something for them.
A lot of the new userbase is on mobile platforms and on tablets. That new UI should be "responsive" (automatically adapting to different screen sized) so that it runs well on tablets and smart phones as well. With Cordova and similar toolkits, we have a technological basis to quickly make a mobile app out it so that it installs like any other app. It would be a replacement for the system "email" app.
The goal for the new UI are 1 billion users.