This has probably been up here before, at least I know it has been up on Guru a few times since way back.
Make bosses less static. Be it the dragon champions, other world bosses, dungeon bosses etc. Instead of them just standing there, throwing attacks at you (or just standing there roaring while you throw your everything at them), make them move around and do different stuff. And with move around, I do not mean them having to move around over the entire zone, but around the smaller area where they spawn.
Look at this video, for example. That boss looks very interesting because it moves around and you have to stay alert to be able to dodge attacks and keep an eye on it all the time to avoid things. This is in a dungeon, but from what I have seen from other videos, they have similar bosses out in the world as well.