Yes! That solution would actually be really good and probably work quite good aswell. The only problem I could see is all the factions getting weak again if you for some reason lose your power status, it would feel really unnatural.
The game would for sure need some stages in the game where it becomes more difficult because even if you cant amas armies without a kingdom you will in a matter of time become so powerfull that you can take on everyone pretty effortlessly.
What i would do (I can't do anything keep in mind) is have a power meter with milestones for the player's non-faction clan,
Say at first it's as it is currently, you take a fief and make peace it's all honky dory. But as you take more fiefs and grow in power you reach milestone where now 1 faction you've taken a fief from or have -relation with ruler can send "foreign mercenaries" to attack you and try to take the fief back, this would be good because it wouldn't spread the faction out like if they sent thier own clan at you,, then eventually a level where a faction can declare war on you, just 1 at a time for now. Then another where up to 2 can declarer war on you, then finally you are the same as a faction so you might as well get some vassals or expect the continent to team up on you soon.
But I would also add other things like vassal coming to you at the last point or a means to contact the with messages so you could actually get them while also defending yourself. I would want this for any faction creation and a short respite time between declaring a faction (if at peace) and being attacked, like you've decided to be a kingdom and are becoming one, but there's a window before the news gets out and you're open for attacks.
I would also want an option for more parties and patrols to, so feasibly you could fight the factions even if you didn't have vassals, because it's a single player game and it's good to have more options. I mean if you have the money and everywhere is packed with soldiers for hire, why can't you just put them to work defending you stuff? Like you made new type of government, you don't have vassals, you have a standing army and generals and stuff, the other factions all start to rally against you..... will you change the world or will your new ideas be wiped out?
Anyways crazy stuff that would be a good mod.
The only thing I might actually mod myself is making the long glaive look like a dragon slayer.