This is a pretty simple request and I don't think it's asking too much, especially since Hydra Chests are a Weekly reward. The problem is, Adept and Warrior Chest on Normal and Guardian Chest on Brutal both drop 1-2 Mortal Soulstones. The best is Grandmaster Chest on Nightmare with 1-3 Mortal Soulstones per chest. In fact it's the ONLY chest that has the chance of 3 Mortal Soulstones...

Now I'm assuming that the chance of 2 Soulstones is waaaay higher on Brutal chests than Normal ones, but given the importance of Souls in all areas of the game now there should be a chance for an Immortal from Brutal and Nightmare's top chests (Guardian and Grandmaster). For simplicity, here's what I think those 2 chests should drop with % chances. If you have different numbers, would love to hear them.

Brutal Guardian Chest: 2-3 Mortal Soulstones and a 5-10% chance of 1 Immortal Soulstone

Nightmare Grand Master Chest: 3-4 Mortal Soulstones and a 20-30% Chance of 1 Immortal Soulstone and 5% chance of 2 Immortal Soulstones

Also, once people get Mithrala it would be very nice to start getting 1 Fragment per chest on Hard-Nightmare that can be used for Maud