To be fair that is already the case.
In D:OS there were quests that you would get the smallest amount of XP by just killing the guy. Larger if you bring them an item or other NPC which in turn was locked behind a speech check. (There is one with a troll for example)
That´s not true!
http://www.divinityoriginalsin.com/usergen/ivar_walkthrough.pdfPage 52: Victoria / EglandaerKill Victoria = 2745 XP
Kill Eglandaer = 2550 XP
Fool Eglandaer = 2400 XP
Arrest Eglandaer = 2100 XP
Page 55: Guards at the BridgeResist Order and Kill the legionnaires = 1080 XP
Accept the orders = 900 XP
Page 101/102: Looting the Warehouse (Cyseal Task)Fight the guards = 2050 XP
Door is opened = 260 XP
Sneak inside = 0XP
Just to name a few examples 