1: Give weapons a certain skill, for example you have a dagger and that dagger has a skill named shadow dagger. This will let the shadow of the dagger attack too. The skill will only work for a limited time and it won't be useable for a certain amount of time. Another example is you have something like lightning on your weapon. This will lightning will spread when you attack an enemy.
2: A line of quests. So here you have like 5 quests in a row with different choices and if you pick the right ones you will get a higher reward.
3: ( Secret ) duo quests. So what I had in mind of this is when you are with someone and you get a quest for you guys. It will be a line of quests just like above, but when your'e halfway through you get the opportunity to betray your quest buddy, and fight him too get all the rewards. And maybe if you both deny to betray you will get an even higher reward.
Remember these are just some ideas. I want to know your thoughts on this.