Dracula? More than Dracula? Well yes, if Gannon was next door and...
Well, the thing with Bongo Bongo was that he was just a dungeon boss. He came about maybe 1-2 hours of gameplay after the boss before him, and there was another boss bout 1-2 hours after him. Max. Nobody said anything about him before you met him that I recall, and you're right that nobody ever talked about him afterward. And worse yet, killing him didn't save anybody or accomplish anything except getting you the next Sage freed that you needed to take on the real boss of the game, Gannon. It's those things that made Bongo Bongo just a boss, and not an archvillain.
With AVWW, we have minor "named monster" outposts, too. Those are somewhat like Bongo Bongo. Nobody talks about them overmuch except to tell you the exist when they ask you to get rid of them, and getting rid of those smaller named monsters only solves a problem in a local area.
On the other hand, with the evil overlords we're talking about for AVWW, you're going to have to play for 10-20+ hours to bring down one of those fellas. Doing so requires not only the fight with the overlord themselves, but also traversing their entire fortress of bad guys that they have on hand to protect said fortress. Prior to defeating that bad guy, you'll have 10-20 hours of NPCs in a wide range of regions who are lamenting the crushing yoke of this overlord in various ways. And if you try to attack the overlord too early, after even finding their keep to begin with, expect to die. Once you finally do defeat the overlord, the game remembers that forever (that's part of the "deeds" system we've talked about in the past). To people that were affected by the overlord in the first place, this is a really big deal and they remember both the overlord and the specific character that defeated him/her.
It's all in how you build up the overlord, I'm convinced. It's kind of like the way AI War builds up to the final fights at the procedurally-generated AI homeworlds. There's a strategic component to even figuring out how to defeat each overlord, because each one is outfitted so differently. So you might have to do some recon and some refitting as you go, too, in order to even take them out.