3. Vaults don't really make the game harder... They're avoidable, so you can just do the easy ones and skip the ones you don't want to do. They give lots of experience and items.
4. Implemented already to an extent via
1116. I haven't played enough endgame characters to have a good sense of how strong this effect is, but from what little I've seen, there is a definite improvement.
5. Not a bad idea.
6. Monsters don't have inventories. Having them use artifacts is a problem if they can drop those artifacts, anyway. Monsters with access to piety would be cool though.
7. Won't work as well as you'd expect, since an endgame PC is faster than most monsters. They'll spend all their time running away and you can hit them from behind. Also, with the exception of titans and a few other monsters, most monster ranged attacks are quite pitiful compared to their melee.
8. Monsters don't have inventories, so this won't work without significant changes.
9. Improved monster spellcasting would be cool. I don't know about earthquake specifically, since that spell is kind of lame, but I get the idea. Part of the problem is that a lot of spells that work well for the PC wouldn't work well for monsters. A monster that could acid ball would be interesting, except that it would destroy all of the monsters around it as well. Having a few monsters that could cast Petrification might spice up the endgame a bit though ;)
10. See #8.