> submit ranking code to public github repo, the algo will be played by thousand SEO scammers to their advantage

Just a thought : Ranking code could itself learn & adapt to each individual user (the learned "weights" could be sync'd online across your devices). Weighted signals from users can be fed back to the mother ranking algorithm (un-customized one). Basically millions of distributed deep minds[1], instead of a single one.

I can imagine there are a lot of holes in my theory, but we can't simply accept that open sourcing the algorithm implies that it can't be done.

[1] : https://deepmind.com/