Unfortunately, with the title of this thread it may be difficult to get legit responses from people. A less antagonistic title/OP may have lead to a better discussion on the topic at hand.
Regardless, it is a topic that I am interested in. Really enjoyed the finishing moves from DAO and was disappointed that DA2 replaced them with the exploding bodies. When I say finishing moves, I am referring to the ones against normal human sized enemies. I thought the ones used against the big creatures (ogres/dragons) were too long and drawn out.
However, while I enjoyed seeing the finishing moves, like the decaps and shield bash, they did make your character uncontrollable for a short period of time. So my suggestion would be to add a positive effect if you or a party member perform a finisher mid combat. Maybe a short stamina/mana regen, almost acting as a morale booster to your allies. Making the chance to perform a finisher low enough to where this would not become overpowering.
You could also tie in class skills to this. Maybe an Assassin would have a skill that increases your passive fatality chance by a significant amount. A Champion would have a skill that basically gives your allies attack buffs when performing a finisher, to signify his "leadership" on the battlefield. Maybe a Reaver would have a skill where if he performs a finisher, enemies around him in a short AoE become feared for a short time.
Just brainstorming here and trying to figure out ways to offset the fact that you lose control of your character for a short time. Having beneficial effects proc from finishers would give players a reason to enjoy them, beyond watching a well done animation.
Speaking of animations, I really hope they don't incorporate the horrible ragdolls from the Mass Effect series into DAI whenever you kill an enemy. From what I have seen, they may well be doing that. Crossing my fingers that isn't the case, lol.