UPDATED FOR NEXT GEN

Description:
This mod simply rewards Geralt for being the altruistic hero that he is by saving those poor monsters and gives him the rare Succubus and Doppler mutagens that he deserves. (I know technically that mutagens need to be ripped out of the bloody corpses of said monsters, but hey....the Succubus still manages to reward Geralt with her trophy anyway. So, if she can do that, then she and the doppler can cut a piece of a mutagen out of themselves for him, too. After all, he is letting them live.)

1. Geralt will now receive the Succubus mutagen for letting Salma go in "Deadly Delights" quest.

2. Geralt will now receive the Doppler mutagen AND the Doppler Trophy for letting the Doppler go in "An Elusive Thief" quest. The cool thing about this is that by getting the trophy, (I just assume the doppler had a spare doppler head in his back pocket like the succubus had) you technically "complete" the quest since the trophy is registered in your inventory, and you will get the full contract reward as if you killed the doppler when you turn the monster contract in. Sweet Geralt, lying to spare the Doppler's life that he saved, just like he lied when he let Salma the succubus go. Such a good guy.

I know you can get these rewards through console commands, but there's nothing lore-friendly about that, is there? Plus, I still think it's garbage that you'll be punished and not receive such rare items AND not have the game reward them to you in any other way, other than by being totally evil and slaying these poor monsters. So now, you can safely get these rewards and sleep soundly at night knowing you did the right thing as a good witcher. :)

Optional File:
Added an optional file where you can get the Cave Troll Trophy from Wham-a-Wham if you choose to spare him. He gives it to you as a reward for letting him live. It was salvaged from his brother's corpse, Slam-a-Slam.

Enjoy!

PLEASE NOTE: If you already completed these missions, regardless of saving them or killing them, you will NOT retroactively be rewarded with the mutagens and rewards. You MUST install this mod before completing those quests to get the rewards for saving them. In this case, download the Optional File "Succubus Doppler and Troll Rewards Purchasable" to buy them from the Loan Shark in Novigrad. That guy always gets the most suspicious stuff. Creep...

Installation:
Simply extract, then drag-and-drop into your mods folder. Might need merging if you have other mods that modify the same file:
gameplay/rewards/rewards.xml