So perhaps there could be a system where a member of a faction could sue for peace independently, like in CK and EU, or even offer to "defect" to the winning faction (like an Axis minor declaring war on Germany), but then the leader of the faction has x days to occupy, say, 50% of their victory points, and that allows them to... I don't know, start a civil war or something?
Kind of.
I was thinking of something like this (for AI nations only):
1) Rework war support mechanics so that it responds to overall casualties and loss of cores. If necessary, boost war support for certain majors like the Soviets. Higher conscription laws might also factor into this.
2) Establish specific "country considers leaving faction" thresholds for ruling party support and war support. The lower the war support, and the lower ruling party popularity is, the worse the new effects are. (Also, rework ruling party support in various countries so that none of them are 90% or higher for things like fascism at game start.)
3) At each threshold of war support and ruling party popularity, assign some kind of "bad events/ticking stability loss" events and/or mechanic. Each of these events or ticking stability loss event fires a step in the "we are defecting from our faction" mechanic. I'd see four steps in such a mechanic (kind of like how puppets have different steps): All expeditionary forces loaned to faction members returned and all garrison manpower loaned to faction members recalled, diplomatic overtures to enemies, military take over of government/new government formed, attempt to switch sides.
4) During the diplomatic overtures phase, if the opposing faction refuses an offer for the country to change sides, no further progress will be made and the country won't defect. (To use Romania as a historical example, there's no reason for Romania to defect from the Axis if Stalin signals a refusal to recognize King Michael and his government as legitimate.)
5) During new government formed phase, the ruling party may be switched, elections are suspended if the government allows elections, and ministers that encourage ideologies opposed to the current faction are installed automatically and for free.
6) Upon switching sides, the country becomes hostile to its old faction. Territory occupied by its new faction remains occupied until the peace conference.
7) This is important: progressing through the steps can be slowed or stalled by the presence of faction leader troops on the country's VPs. You need more troops on those VPs to stall or slow progress the further the country has progressed in steps. The required troop count also increases based on how poorly supported the ideology of the faction leader is in the country trying to defect and how low war support is.
The goal of the system would be to give countries in HOI4 the historical tools Germany used to try and stop Hungary, Romania, and Italy from defecting. It should be clear to the player that defection is coming (Germany knew what was happening and in each case tried to stop them), and there should be prevention measures. The ideal situation would be that faction leaders on the losing side of the war should have to actually pay attention to how bad the war is going for minors and not just in terms of losing cores.