Well, anyone can become the Emperor. I've either played or seen Ottoman, Oman, Japan and Aztec Emperors, both AI and player. What do you think about adding a parallel to Annex Vassal that turns a vassal into a relations-free subordinate? In an ideal world I'd like them to behave something like a Daimyo rather than a vanilla vassal: mostly fighting each other, but becoming a threat to the suzerain if left unchecked too long.
I also keep permanent or near-permanent vassals who I don't annex until near the end of the game. Burgundy as a vassal is my favorite, more trustworthy and as strong as most allies. Or Shun, if I'm playing around in Asia. Depending on the game, I sometimes just keep strong vassals and forget about allies altogether - and it works often better. Annexing them means I'm ending the game, which makes me feel a bit sad.
Regarding more vassal autonomy: I deliberately ignored Japan unification in my Shi'ite Japan campaign and remained Shogunate for the whole game. Shogunate's +4 relationships and allowing the vassals to fight each other certainly added more depth and made me think twice before getting new vassals. Not being the HRE meant I had to annex them at one point or another, but I like how my vassals screwed things for me.
For example, I conquered, converted and released Khmer in two provinces. Khmer is now a proper Shi'ite, and could take more provinces from Ayutthaya. Too bad, I'm an idiot, I had still Buddhist Dai Viet as a vassal. Dai Viet completely annexed Khmer. Also, I was dumb enough to release one province Sibir next to three provinces Uzbek, with the hopes for future feeding to both, except Sibir got annexed by Uzbeks. Sometimes I could use it as my advantage. I diplovassalized Gujarat who had.. maybe 5 provinces. I force-vassalized next door Hindu Kathiawar, with 2 provinces. I was hoping Gujarat would do the "annexing" for me and no wonder, they soon did. I hated seeing diplovassalized Sunnis conquering and converting my catch-and-release Shi'a vassals but oh well.
Inter-vassal alliance and warfare are probably meant only for Japan and Daimyos. But doing it on a continental scale was fun, and I really hated annexing vassals as their own struggles and autonomy really made me respect them as nations on their own rather than just temporary holders for provinces. I actually liked how they screwed my plans for their own ambitions, and now I really do miss the mechanics as I'm not Shogunate anymore. Perhaps someone could mod it so the inter-vassal warfare is allowed for all government types.
In addition to more autonomous inter-vassal interactions (which will obviously be only for the player nations, since the AIs usually annex vassals as soon as they can.), I would also like for the vassal to request wars for their own behalf. A vassal may decide it wants to reclaim its uncontested cores, and sends a request similar to CtA to its overlord - and instead of Accept / Refuse, the overlord option might be Allow / Forbid. Allowing would mean the overlord joins the war, but the vassal remains the war leader and the vassal can negotiate for themselves in this special case. Forbidding would disallow the vassal to declare war in the first place, but give something like -100 relationshp malus, which would go away in 15 to 25 years depending on your BROT. Restricting some casus belli would be probably necessary (I think Reconquest, Colonialism and Colonial Conquest would be enough), and they might want to take on your ally, causing a strain on the relationship on either one of them.
And lowering the cost for the Independence CB - say, the stability hit for declaring war on the overlord is reduced to a reasonable extent while the annexation is taking place or the vassal's opinion of you falls below a certain limit. You'll be keen on the integration process and might want to be ready for a potential war with your vassal. So that if you're annexing several vassals at the same time, they might decide to ally each other and DoW you for independence. This will make the standard vassal feeding strategy a bit more risky, but I think the game would be more interesting that way.
Similarly... I don't think I cored anything myself, this was with Denmark. Vassals are the only way
Umm, I think you're coring two provinces in Tibet on that screenshot.
