What qualifications do you propose for mastery 8?

Yes.

This is actually a great idea. It would allow for many challenges in a game without going out of the normal playstyle. You'd get things like "Cash in x Adoration stacks in a game, catch x Axes in succession etc." with Draven, "Have x Turrets be taken by Yorick / Maiden of the Mist, trap 2+ enemies in 1 W etc" for Yorick or "Have x Groves active at the same time, use x brushes to deny in combat vision for x seconds (x times) or heal your allies for x total" with Ivern.

Would be interesting though, because for champs like Rammus, I can't think of anything.

Same I was thinking. We don't really need more mastery stupid things but they should be things like the Overwatch challenges like boop 4 poeple of the map with Pharah a number of times. It'd prove you can replicate your success with said champion consistently.

Maybe 50k Mastery points, then a 'quest' depending on the champion itself and a few S+ tokens, maybe three. 350k Mastery is a bit much...

The higher levels should be reserved for mains. Maybe level 10 instead of 8, but I'm riding on over 500k mastery on my main here. 50k is achieved just by getting level 7 in the first place.