I think just the history->xml conversion would be a bit of a hard sell, but I would be willing to pay for a sub if there were some additional features like:

* easy add to my filter list (like maybe a browser plugin so I can see that the current site isn't in my filter, but I can click a button and now it's in my filter & opposite for remove for when sites start to suck)

* stats on which sites I visit after searching

* aggregate bing+google filtered searches

* curated site lists for different topics, top 100, etc. Maybe like a temporary search using these sets such that I can try them without affecting my own filters. Maybe sharing lists w friends

* some sort of search anonymization/log deletion feature

* integration with browser search on desktop & mobile

* search flags like duckduckgo so I can easily switch filter sets by typing like /news or /nerdshit in the query

* integration with archive.ph & wayback machine