A Few Recommendations
Proactivity over Reactivity
This is mainly directed at the idea of having a main antagonist (an idea which, when done right, has the potential to be, as many others have stated before me, a tremendous success). Rather than having a villain that is reactive (taking on the role of antagonist simply to fill an OOC gap or because the races of LoTC have 'irked' them in some form or another) is far inferior to a proactive (a villain with motives, set to be acheieved regardless of our action or inaction) one. Immersion is bolstered with the feeling that we are, as we should feel, in a living world, and our nations and races are only pieces on the board, as the world moves around us, rather than for us. Frankly, if we get in the way of some hellish being's machinations, only then will they begin to care. Supervillains would much prefer a world without superheroes, they might finally be able to finish the job.
Conflict
Conflict is the cornerstone of any storyline - it's what gets you on the edge of your seat and roots you there permanently until you realise you've been holding your breath if done right. Of course, conflict for conflict's sake can achieve the opposite and become dull, so we need a reason just as much. The conflict could be between nations (what comes to mind for me is the Cold War - the epitome of 'edge-of-your-seat' with paranoia, espionage and supposed pre-emptive movements creating divides between two superpowers with the scariest weaponry ever seen) or with a major antagonist, although they wouldn't create conflict for the sake of it (however, I am aware that there is a need to create interesting events for everyone) and would be far more surgical about their moves, which leads into my next point...
Fear
The best way to keep someone firmly focused on something is by scaring them to the point where they can't look away. In a game built around punching trees, it's hard to do, but possible. Rather than rumours and whispers of a villain's power, a demonstration does the job just as well. Players need to be terrified into believing that there is something out there that could strike at any time, any place, and make you nervous to even log in. Mads Mikkelsen's Hannibal Lecter in the recent TV adaptation is an example of how villains (yes I'm romanticising the idea of an antagonist, but please continue to humour me) can be terrifying, whilst still being vulnerable. The man himself bases his life on subterfuge (a technique I'm pleased to see others suggesting), and an uncanny ability to manipulate and plan ahead. As a viewer, I can only say how riveting it was to see a man, as vulnerable as any other, be so far ahead of everyone else, and so well-prepared, that even I was second-guessing the heroes' movements. When he finally revealed himself, he did so in a way that crippled the heroes immeasurably, and shrank any chance of potential resistance. That's what we need.
OOC Interest
It's all good and well me harping on about how we need this and that, but the bottom line is it needs to appeal to players. Whatever is decided on, people need to be drawn in, motovated to log in and find out what is happening. In this case, I suggest utmost secrecy, with not even any OOC knowledge or disclosure about the storyline being made - only intrigue and initiative will yield the truth. Having your character not know something, but you knowing it makes it slightly interesting. Making you know as little as your on-screen blockhead is massively interesting. It creates doubt, confusion and intrigue, the likes of which are difficult to replicate. On the subject of involvement, getting players stuck in is also important: forcing us into acting by trapping us between a rock and a hard place opens up a load of possibilities. Maybe we are the rock, the hard place, the one forcing us in between and the one in between all at once. Maybe the antagonist is already out there right now, hidden inside the body of a nooblet. All jokes aside, watching a story develop is far better than reading about how event-based characters are making forceful developments.
Final Words
I believe a main antagonist is the way to go, but for the love of God, make us fear them. Do the unthinkable; wipe out a nation, a race, hell, even destroy a whole isle. Force people towards each other, see what cracks appear and act on them. Maybe even make use lose for once - no more of this escaping; if you wan't to get away to the next map, then overthrow the new overlord/s and lead a revolution. We only leave when we earn it. It gives us an IC and an OOC reason to log in, fight, and move on. After almost five years, it's time we shook things up a bit. If you can get an OOC reaction, you get an IC reaction (maybe wrecking some fast travel points is a good idea to start with, that'll piss people off, and stop us 'escaping' to 5.0 as easily, plus people will want to get it back OOCly). I trust you to make us care about a major story that touches everyone, not some sideline tale that's group-restricted.