Why would any of this be good, much less the combination of them? You are basically suggesting a system where everyone buys big ai armies all the time and prays that they do something useful and win the war, while most likely constantly getting frustrated as they proceed to do all the janky stuff the ai tends to get up to.
Setting aside the act like vassal armies bit for a moment, lets look at the “Don’t count against force limit” and “are much cheaper” bits. The merc armies aren’t exactly cheap right now, but they also aren’t super expensive. A country can generally hire most if not all of the mercenary companies available to them entierly on loans and still have a pretty large amount of room between them and bankruptcy. Currently, doing that sort of thing generally gets you absolutely slammed by going over forcelimit, making the already high maintenance utterly unsustainable. If the only change made is making them not count against force limit, then that already makes it so that a country of some 20-30 dev could sustain an army of 20-30k for a significant amount of time assuming they have the companies available, which in europe they definitely do. That is utterly bonkers. If you then also make them significantly cheaper, then there really isn’t all that much stopping the player from say, building up a huge merc army and straight up soloing the plc as lubec or something. Unless of course the ai does the same, which would make the whole thing just a huge mess.
Which brings me back to the first thing, the “act like vassals” bit, and the ai control I am assuming goes with it. If you want that limitation to act as a counter balance to the absurd power they would give from the other suggestions, then I will tell you that is an *awful* way to do it. Mercs are the tool you tend to go for when a war is going to be really rough or is already going badly. These are the situations where the ai is most likely to act up as they try to preserve their forces against a strong foe, and the situation where the ai being non cooperative leads to the most player pain, as it leads to lost wars and possible game over. For vassals and allies, this is usually ok since the player can be in those roles as well, and so any extra control over them would lead to either rule differences between humans and ai, or humans being screwed by the ai controlling them. Its also entirely expected for both those groups to have their own agendas and thus be less than fully cooperative. Mercs on the other hand gameplay wise exist to serve the player and nothing else. If I hire them, I want and expect them to do exactly what I want them to, no matter the cost.
If the goal of that stipulation is just to avoid having to actually fight your own wars, then that is frankly what vassals are for, or maybe some mechanic introduced for the purpose. It is not what I think anyone wants out of what can often be their only lifeline for when things go south.
