Power users accumulate years of email client customizations (rules, shortcuts, plugins) that make switching between clients painful.
Build a toolkit that analyzes a user's current email setup and generates migration scripts for new clients, preserving rules, signatures, and keyboard shortcuts.
Charge power users who need to switch for work or privacy reasons.
Start with Thunderbird-to-other migrations since its user base is tech-savvy.
Risk is email clients changing their extension APIs frequently.