Update me please?

Slark is still hard to deal with in high level games right? His ultimate is 4 seconds of complete invisibility, and he can do whatever he wants during these 4 seconds while being healed. Imo the only way to deal with him is to chainstun him before he kills you.

How big are the changes to Doom, Death Prophet and Riki? I've noticed that they have new skills. Do these new skills heavily effect how you play these heroes?

Uhm....you can do a lot to Slark when he uses Shadow Dance.
Aoe slows like Crystal Nova, aoe stuns/taunts ( Berserker's Call) or just silence him.
He really suffers when being unable to pick people of so a casual Ghost Sceptre goes a long way and of course sticking together and having sentries with you when you push.

Michi said it! Basically Shadow Dance makes you invisible, not completely invulnerable, everything AoE still affects you.

Note here what the pick and winrate for Slark are in over 5k:
https://www.dotabuff.com/heroes/meta = 26.82%/49.75%

As a general indication, hero picks and hero winrates below 50% mean the hero is somewhat balanced, since otherwise he would be constantly picked and win more than half the times he is picked.