The latest Live Letter pretty much implied that some of the theories regarding Regula are correct and that during the Azyz Lla side quests in 3.3 he is going to become corrupted somehow. With that in mind - and the fact that this will be the third time we face him directly - I do wonder if he'll be 4.0's main antagonist or at least one of the expansions' main antagonists. Alternatively he could just be dealt with by the end of the Triad storyline but it seems like 3.4 will have us fight one of the two remaining Eikons whilst the other will likely be defeated in 3.5.
...which is, incidentally, right before 4.0! So either the entire storyline gets wrapped up by then or it plays a part in causing us to go to Ala Mhigo. Wasn't there a mention of the next expansion potentially involving water and a flood? I could see the Triad's power being turned on Ala Mhigo and the surrounding areas to justify us going there.