I had a passing thought while I was out for a walk earlier today about the status of the Sunstrider Family and Lorthe'mar firmly turning down repeated proposals to become King of Quel'Thalas by default. I know he's said he doesn't want the title of King and avoids sitting on the throne as a statement but no one would both pass on something like that AND not step down as Regent Lord without a really good reason.
I have three running theories of why and a huge problem with all of them:
Advisors Need to Vote on a New Permanent Leader
If I remember right, Lorthe'mar told Halduron he'll happily step down once the powers that be choose someone to become Ruler of Silvermoon. That was waaaaay back after the end of Burning Crusade. The thing is, it's been about 5 or 8 years since then. I can understand partisan infighting and disagreements but 5+ years later and still no new Leader makes no sense. I did account for Kael'Thas' decree his father Anasterian would be the last King of Silvermoon in name so another title like Emperor or something could be used.
A Sunstrider Family Member survived the Scourge Invasion + The Burning Crusade
It's certainly a possability and I scoured Wowpedia to see if I may have overlooked a Sunstrider other than Kael'Thas and Lyandra (Undead and Killable during Fire Mage Artifact Quest). If Anasterian had any children, siblings, nieces/nephews other than Kael'Thas at the time of the Scourge Invasion, we don't know. Assuming he did, the obvious next question is did any (aside from Kael'Thas and Lyandra) survive which again, we don't know. Too many unknown unknowns there.
Blizz left this area ambiguous unintentionally but now, it could be used to pull a long lost Sunstrider out of thin air and it would be believeble. If this actually does happen, Blizz can explain their absence prior to their WoW debut in some way that would be pretty understandable but would fit with known lore. They would have to have survived the Scourge Invasion of Quel'Thalas obviously. Which means they either weren't there at the time like Kael'Thas (he was in Dalaran), they kept hidden or they were long thought dead. As long as it makes sense and fits the timeline, it could work. Hell, it could be revealed a Sunstrider is among the Void Elves for all we know. I assume records of the Sunstrider Family line is well documented in Silvermoon City.
There is a living heir to the throne but s/he is not yet of age
Not quite the same as above though again, it would have to be someone previously unknown. A child who could claim the throne would preferably be sired by Anasterian, Kael'Thas or another currently unknown member of the main family. A branch family member could claim the throne by default like Lyandra was looking to do when the right conditions are met. Like above, they would have to have survived the Scourge Invasion.
They would be recorded somewhere in Silvermoon City though there is one situation where that might not happen: If the child is a illegitimate. As in the child's mother is a concubine, prostitute, commoner or non-Sindorei. Chances of that last one would be pretty slim after The Second War and even more so after the Scourge Invasion. Royals living away from their homeland for an extended period tend to either renounce their claim to the throne, want nothing to do with that life or were exiled (and their claim to the throne stripped). So even if they had a child, unless the parent still retained claim to the throne it wouldn't matter.
Usually in an illegitimate child situation though, the mother either flees the country and keeps the identity of the child's father a closely guarded secret to protect their life or abandons the child to protect them both. If Anasterian had any kids a few years before he died, we don't know. There's no evidence suggesting Kael'Thas and Jaina were serious while he was in Dalaran and he was too busy trying to lead his people after the Scourage sacked Quel'Thalas to think about much else. Same when he joined up with Illidan in Outland. Plus he got killed in TK and is Fel-rezzed by Kil'Jaeden. If Anasterian had any other children where were not of age when he died, we simply don't know. If they survived the Scourge Invasion, they'd be of age now though. There's no guarantee Lorthe'mar would know either.
...Those are the three theories plus the glaring problems with all possability.
If the first one happens, Lorthe'mar would certainly be happy to step aside. As for the other two, enough time has passed that I think the Sindorei would welcome a long lost Sunstrider. More so if it's someone known, liked and long believed dead. A child of Kael would need to earn trust. A child of any other royal would be welcomed and would have to prove they're worthy of the throne. An illegitimate child, however...THAT would be some GOOD Fan Fiction to read! Another interesting angle: A Sunstrider family member or heir who is loyal to the Alliance ala Alleria and Vereesa of the Windrunner Sisters. I would need some popcorn for that!
Anywho, that's where I'm at. Believe it or not, I resisted the urge to let my head canon run wild. If there is an heir or a surviving Sunstrider out there, personally I do suspect Lorthe'mar may actually be aware and is simply waiting for the right time for them to be introduced. Nothing but speculation on my part though.