There are about as many different email workflows as there are users. Everyone has slightly different needs. Rather than try to come up with one configuration and collection of settings to satisfy all users, Mailspring should aim to be a hackable email client.
Plugin Catalog: Discover, install, update, and toggle plugins individually, including all core plugins.
Theme Catalog: Discover install, and update themes.
Provide in-app JSON-based configuration and keymap editing.
Provide in-app JSON-based advanced account configuration.
Onboarding flow should allow the user to select sets of starter plugins based on their workflow.
Allow exporting/importing mail rules.
Flatpak distribution (better Linux coverage, less bugs).
Improving theme and plugin templates.
Plugin and theme development documentation.
(Proposed) Expand mail rules?
(Proposed) Allow syncing encrypted config file, keymap, and/or mail rules over Mailspring ID.