A few things...
1. I agree that long-term stable alliances that effect no changes tend to be pretty boring, especially for new players. I don't think the SA bloc is a problem because they've been steadily spreading and they're not just refighting the same wars all over again (and what Arcaea-bloc are you talking about, anyway? We've got one ally that we can count on with any certainty at the moment, with the rest of the continent either at war with or neutral in the conflict right now) like, for instance, the Sirion/Fontan conflict. I think the situation on Atamara is a bit anomalous because the CE bloc clearly was making changes, and somehow no coalition united against their expanding influence until very recently. Why the current war isn't drawing more people I'm not sure, which leads to the next point...
2. Positions. After playing the game for several years and watching what happens in various realms, I'm convinced that it's not so much position turnover (though having some is of course required) as position (and influence) availability that draws people. I'm pretty confident in saying that Arcaea's one Lordship/Council seat per family rule has been the single biggest reason for its success, closely followed by more luck than I can shake a stick at and the general policy of just accepting that spies exist and telling people stuff anyway. All else being equal, the realms I've been in with position restrictions have had a much more happy and involved playerbase than those that haven't.
Now, as I alluded to, this isn't just game mechanic positions but opportunities to influence. Fear of spies (or factionalism) has ended up crippling a lot of places by making it so they don't share anything outside of the inner circle, and that leads to confused and bored people who don't think they have any say in the realm. People who don't think they have an opportunity to have a say tend to leave.
3. Gold. This one's pretty basic, but places that make an effort to get new characters reasonable oaths, as opposed to doing these godawful "trial oaths" at some ridiculous percentage (10% of a crappy rural, really?) are going to do better. This can also be mitigated by some sort of central gold distribution system (for instance, Perdan right now has CS requirements per army, and beyond your oath you get however much gold you need from the General to get a proper unit), but you have to make that really clear in the first few days for it to have much of an effect.
4. More effort making sure new players don't get lost. I'm blackly guilty of this one, as I don't tend to even bother saying anything to a new player until they've actually sent something, and half the time I get distracted and won't say anything even then.
