It would be cool to have a mission/art that reset all your scanned planets except the ones you own...that would be hot.
OR
It purged all enemy controlled planets, and if there were none then it would purge all legion and if there were none, it would purge all unowned planets.

That would be a neat trick, I think.

A big negative to this because of how you set it up. I never want to purge all enemy planets, there are too many reasons to keep them. If it purged a large number of unoccupied I would consider it. But still very doubtful because there are already planet purgers, and I do prefer to decide what planets are purged myself.

There already is a mission that purges your planet database. It's called Stellar Cartography. Plus you get more Purgers from artifact points. Your idea isn't going to happen, sorry.