Consider the issues with these suggestions you're putting forward. First off, Final Boss.

This is actually one I agree with, for there are some trim requirements that you'd have to be atleast knowledgeable about PVM to accomplish (100% Enrage Telos), but don't forget Final Boss requires 100 of each Raid Boss(I still have 30 on each, a year after release), 100 Fight Kilns, etc. Some people just don't enjoy PvM or can't find a team for certain bosses. It would be good to see, though. 

- Well why stop there? Why not 100,000 kills of every mob?

I'm assuming this is a joke, but I'll answer it all the same, though with a more realistic figure of say, 1000. This is possible, but frankly, why? Why bother killing 1000 chickens, goblins, etc. That's just pointless and not challenging. 

- Besides, someone with trim comp would most likely already have over 1k kc of most of the higher up slayer monsters. 

Can confirm;

14634 Airut

41153 Abbysal Demons

18343 Edimmu

29574 Dark Beasts

7547 Muspah

I would say these are the main 5 for high-level slayer.. But, as 120 Slayer will be a requirement for Completionist Cape next year, you may see this becoming a reality in some way. 

- What does playing castle wars but not other minigames complete? why not make soulwars jad pet one requirement?

I don't really have an answer for this. I could say that it's because it's just a cosmetic unlockable.. But so is full Decorative Profound.. 

That said, Soul Wars is extremely dead, so you'd have to Thaler it. And Thaler was a failure, so I don't see them taking this direction. 

- Completing every possible Slayer task would at least make more sense

Why? Why would I want to kill 65 bears, 42 Werewolves, etc. Just a pointless grind with nothing to show for it.

If you want a real requirement, add getting Divine Storm unlocked to the list. It's not a req.