So interestingly I think "better spam detection" has a relatively straightforward interface (e.g. connect this app to Gmail's servers via SMTP, move emails between folders), but I assume you need almost Google-scale (or at least more than startup-scale) metadata and spam report collection to be able to do half as well.

One of the rare areas where barrier to innovation isn't market access but actually product quality. That being said I wouldn't pay a lot for this as a standalone service, though I would definitely pay for a standalone email client at this point.