Huh, now I noticed it, China in game never seems to do Tangplosions. How stable is China supposed to be?
Under a decent (not necessarily
great) emperor and without any major issues (e.g. massive Black Death outbreak, successful foreign invasion, or widespread devastation)? At least as stable as any other realm (possibly more so, if they can manage to get a Mandate rating above Average). Under a less good emperor and with something major going wrong? It should be reasonably likely for them to get widespread popular uprisings that risk causing the loss of whole kingdoms, Independence factions, claimant factions, and/or Takeover factions, and if things
start to go poorly they can easily get a lot
worse due to many parts of the Mandate calculation being affected and that fuelling other things (e.g. a disease outbreak results in devastation, hurting the Mandate rating and leaving China weaker, meaning a foreign invasion might succeed, which hurts realm size (and possibly adds more devastation), which hurt the Mandate more, which might cause a popular uprising, which might cause yet more land to be lost, which hurts the Mandate again, which leaves the emperor weak enough that an Independence faction issues an ultimatum...).
Based on the observer games I've run, it's possible for things to go in a lot of different ways even looking at the same start date. Sometimes China gets really unlucky (e.g. they get civil wars due to failed revokations/imprisonments and things spiral out of control) and collapses (or gets taken over by another dynasty, that may or may not be able to get things back under control quickly enough to avoid the same fate). Sometimes it is neither successful nor unsuccessful. Sometimes it grows noticeably (even though I keep JD CBs off) and remains stable.
It should also be noted that the Mingsplosion (or Tangsplosion, or whatever) outcome isn't necessarily what you should expect. China
has collapsed in such a fashion at several points historically, but several dynasties have ended in other ways (e.g. Sui fell to a fairly short civil war and was immediately succeeded by Tang, while Song fell to foreign invasion (a
long time after things started going poorly for them)), so factions overthrowing the ruling dynasty or foreign invasions putting another dynasty on the Dragon Throne are also outcomes that you could expect when China is doing poorly. Also, sometimes a dynasty has managed to endure for a long time after things started looking grim (e.g. Tang not collapsing close to the An-Shi Rebellion and Song enduring well after the Jingkang Incident and the Treaty of Shaoxing), so it's not entirely unreasonable for China to sometimes survive a setback or crisis under the same dynasty (and possibly even the same emperor, even of getting rid of a bad one certainly might help).
So where can I find the Wu Zetian bloodline now?
I'm unsure if it's still around in 769. The only person who could pass it on is Princess Taiping, and I'm unsure if either of her daughters are in the game (and, considering Princess Taiping died in 713, her daughters would be too old to have children by 769, so you'd need to find one of
their daughters (if they had any) or even matrilineal granddaughters (if they had any) to pass it on any further). Much like the bloodline for Wen of Sui, it's mainly there for completeness' sake as things stand, and if you want to get a Chinese Imperial bloodline into your dynasty in the early starts you should probably try to matri-marry some distant member of the Li family instead (or maybe someone from the Sakanoue clan...).
How are adoptions handled in Tianxia? Speaking of adoption, IIRC people discussed having the Tenno adopted a successor from one of the Minamoto-ed (forgive me for lack of a better word) branches of the Imperial House, so can a Minamoto be adopted back into the Imperial House? And can we Minamoto away entire branches of the Yamato house, as opposed to individual characters?
We don't have them (though certain historical characters are scripted to get the dynasty they were adopted into where relevant), and I don't really think they should be added, both because we can't add them where they might be relevant in vanilla (e.g. for Roman culture characters) since we don't mess with vanilla to that extent and because they undermine large parts of the game (the risk of game overs due to your dynasty dying out, the risk of getting a bad heir (or a heir in the wrong kind of marriage, or a heir with an unwanted religion, etc.), bloodline inheritance, etc.). I'm quite aware that they were a thing in the Far East, but some concessions need to be made to balance and overall gameplay.
I have considered the specific case of the Imperial Family adopting back Minamotos (and Tairas), but it has been
quite rare historically, it'd risk causing unwanted issues (e.g. someone's heir being adopted, or even a ruler), it'd make the Tenno's already rather powerful (at least if the Regent isn't meddling) succession law even stronger (just adopt someone good if you don't already have a decent heir!), and it'd of course get very messy to handle cases where someone is a (male line) imperial descendant but
not part of the correct dynasty (e.g. descendants from matrilineal marriages, bastard dynasties, or scripted descendants that took other surnames (e.g. Ashikaga)), and I currently feel it's not really worth doing or would have to be severely restricted (e.g. only allowing it to be used on your own children, grandchildren, siblings, aunts/uncles, cousins, and nieces/nephews, and only if they're not in line for any titles).
Regarding cutting off entire branches of the Imperial Family at once, it makes sense to do that, but it's somewhat messy from a scripting perspective since those descendants potentially could include the Tenno himself if his dynastic parent is alive (at least, I don't believe the decision prevents targeting your own ancestors if they're alive) and just which descendants should be affected could be messy in other cases (e.g. if a kinswoman of the Tenno is matrilineally married to a kinsman of the Tenno and you make the kinsman a Minamoto, what happens to their children? If the children are married to other members of the Imperial Family, what happens to any of
their children?). It might be doable, but I'm unsure if it's too much trouble for the benefit it'd bring.