It doesn't need to have all the "AI" that Superhuman has but keyboard shortcuts and hotkeys are a must. Must also have a mobile app
I've tried Spark, Shortwave, Apple Mail, Gmail, Thunderbird and none of them really compare.
When surveying potential users I discovered that most are happy with the web gmail interface and don't really care. It sounds like a very niche requirement and it'll be very challenging to find enough customers (1k+) who would care about using a very nice email client.