I used some elements of boss battles from MMOs in a D&D game I ran a few years ago that really went over well with the players and helped even the action advantage of a group of PCs vs One Big Bad. D&D 5e did something right with Lair Actions for their legendary monsters, but what happens if the PCs are not fighting legendary monsters, but simply a named villain? Legendary Monsters often have a Lair that 5e treated as kind of a separate character that could only be defeated when the main monster was defeated (or if you removed the monster from the lair). The Lair always acts on Initiative Count 20, can do one of three different things (usually an attack of some kind, alter the environment in some way, and possibly some kind of a defense). The entire premise is that the lair is under the monster's influence because it's lived there for a long time.
What I ended up doing is giving the named villains two initiative counts and the second initiative count would go away at half hit points. The villain could only either attack or move on it's second initiative count. I usually had them move, which kept the villain from getting boxed in by the players. The villain would change some environmental variable, remove itself from a surround, or use magic to escape to another place. It kept the battles constantly moving and the kept the PCs thinking and engaged.
Now, I'd like to preface my suggestion with the fact that I haven't run or played in DFRPG in a couple of years, so I'm a little rusty on rules. If I were to do something like what I'm suggesting in DFRPG, I would build a custom "Boss" power. It would have it's own stress track and would essentially act like another character in the battle, one that goes off of a set initiative based on the level of "Boss" power that's initially purchased. As the power is used it would tick off a box of stress. Once all the boxes are ticked off, the Boss power would go into hibernation. Before the Boss Power is used up, it would certainly make things very interesting for the PCs and should be enough to make the PCs expend some serious resources. It would, in my opinion, provide for a more epic battle. Minions are also very useful, but sometimes it's even more fitting to have someone who could take on three or four PCs just by himself as a show of power.
If I can find my PDFs, I'll try to write something up just to see what it'll look like.