^this. Since we probably won't see ZOS develop any dlc/chapter about the timeline push forward before the whole map is filled (which at this rate will take probably 10 years at least), we can only guess what will happen

Here is my guess:
1.Aldmeri Dominion. I wouldn't call AD alliance unstable, however, the only reason this alliance exist in the first place is its leader, Queen Ayreen. Khajiit joined in because of they own her personal debt, Aeradan Camoran joined in because she helped repel colovian invasion. And now that Aeradan has seen her "true form" in the orrery, silvnar restored, and new mane is selected (Shazah!!), it seems the AD is at its peak.
To me, the likely reasons for AD to fall are:
1)Death of Ayreen.(Probably the only way for AD to fall) She is young even for men standard (28), so she won't be dead by natural causes anytime soon. However, altmer supremacists will always be a problem. Tho I believe most altmers won't go that desperate to seek ally in daedric help like the veiled queen, we can see that there are plenty of altmers in AD considered themselves better than bosmer and "beastmer". Assasination is quite likely when a queen so young and already taking unusual actions.
With altmer's magic and a fully functioning dark brotherhood (unlike in skyrim), the death of Ayreen is possible, and everything will fall shortly after that: the line of "of Alinor" is gone, inheritance will be a mess with both Ayreen and Naemon dead, and the new altmer leader will probably be someone with "popular thought among altmer ppl", meaning whoever that is will probably care less about the other races. While bosmer probably will have to rely on altmer to help defend their province, they can be viewed as "lesser mer" like in the third AD therefore losing the alliance statue, and khajiit doesn't really have a reason to stay in AD after Ayreen is dead.
So, tho this is probably the only way for AD to fall, it is likely and nothing can save AD soon as Ayreen is gone.
2.Ebonheart pact. The most politically unstable alliance among three. Also, tho supposely they have the biggest land mass, both dunmer and argonian are not fully committed to the cause--- House Telvanni openly refused to join in the pact, and only murikmire, shadowfen and thornmarsh tribes have join in the pact. And tho great moot has decided the decision of EP leader must be agreed by other racial 2 leaders, it is actually not clear about who leads their entire race in EP, so again the "Great Moor Government" still need improvement to be a fully functioning government
To me, the likely reasons for EP to fall are:
1)Drop in power/Diminished pro-Pact dunmers. House like Telvanni openly refuse to join in the pact, and in morrowind, we can see many dunmers still are not used to the life without argonian slaves, I don't doubt even the most pro-Pact houses have members who can't wait to get out of the pact, and you never know with dunmeri politics. Once the voices of against-Pact have taken control the houses, dunmers will likely to leave as a whole racial group (tho probably many indivisuals will stay for some time), because honestly without Akaviri, dunmers can maintain their independence easily with living gods' presence.
2)Unfair moot decisions that triggered Argonian/Dunmer. EP leader's decision must be agreed by other two leaders, and if something happens that need immediate decision (eg full sacle Argonian slave revolt), and the leaders cannot reach an agreement, the aftermath will probably lead to one race to leave the pact, and with one gone, the other two probably won't feel the need to band toghter anymore, considering the goal of the pact is to ensure independence, with one race gone, and feud of remaining two races will be magnified and break out. Or even when the leaders have an agreement, the non-EP great houses might try to sabotage the alliance if the decision of great moot affects them.
Unlike AD, EP probably won't have inheritance problem. no matter whether pro-EP argonian/dunmer or price Irnskar take the leadership after Jorunn is dead (as long as not Fildgor, if you saved him), the leadership will most likely band together if they are able to solve the problem within their own racial group.
3.Daggerfall Convenant. I'd say the most stable alliance there is. The city states of breton seems loyal to Emeric and the cause, and with marriage tie of Fahara'jad, redguards doesn't seem to be leaving the convenant anytime soon. Tho Kurog, the original orsinium leader is dead, King Bazrag doesn't seem to be eager to leave the alliance, tho he stated new negotiation is needed. Many independent orc clans have joined in the alliance like seamount clan, and orsinium probably still need the protection of convenant from reachmen (at least, rather than getting attack from both sides), three races have band together despite the odds. DC also has a relatively clear line of succession, Emeric-Fahara'jad-Emeric son (who is half redguard), that will last for a while. Also, DC is a large reason why the other alliances exist at all---EP exist to resist conquer from possible future empire (which is the goal of DC), AD exist because they don't want men to have the throne (tho, honestly, while Tiber Septim proves that altmers may eventually accept a men leader, most of other races will probably not accept an elven supremacist emperor).
To me, the likely reason for DC to fall are:
1)Crowns win. Fahara'jad and his loyal redguards are mostly forebears, with open minds that don't always stick to the isolated Yokudan way. However if the threat of Molag bal is cleared, the truth between crowns and forebears may not last long. Tho with DC's help, forebears will probably not lose a civil war, but if it happens, at least half of the hammerfell will leave the convenant (still, probably not something will destroy DC enitrely, but it is a possibility)
2)Some other malacath-worshipping orc rasie to kinghood. Bazrag doesn't agree to Kurog's trinimac way, but still he understand the reason to band orcs together, and he does not repel the trinimac worshippers entirely. King Bazrag is the racial leader DC needed in order to keep orisinum orcs in the alliance. So if Bazrag is killed in the banner war or another war against reachmen, or the reachmen are dealt with and tension between orcs and breton/redguard intensifies, orsinium orcs will leave the convenant.
These two reasons, depends on how the situation unfolds, may not lead to the total destruction of convenant, and also there are two big "if" when I said about repel molag bal and dealt reachmen. Also, actually the downfall of DC may very well lead to the downfall of other alliances---no man-emperor in a while may lead to altmer supremacist focus more on inner political fight and weaken/destroy AD (still need Ayreen died tho), and EP will disband as soon as their independence is no longer threaten.
And finally....
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FOR THE CONVENANT!!!