For the first time kill with that character, you should definitely get 100% guaranteed loot. Repeated kills for farming should then be made as random drops thereafter. Setting the difficulty higher, first time kills again gives loot and goes back to random when you have it on farm. That's how I see it.
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This is a great idea. I like your thinking. It allows for you to celebrate your first run with cool new engrams but afterwards the rewards get randomised. Perhaps allow the game to have a hidden luck percentage.