(using neutral pronouns for Romeo, Juliet and Rosaline as in "idk that person's gender" because i can) (also i jumped out of chronological order at some point, ask me if you want me to put numbers to show the real chronology or explain stuff)
so basically:
- "rival" houses/political ennemies Montaigue and Capulet
- Romeo and Juliet are the respective heirexs of their houses
- idk if Juliet and Tybalt and Rosaline are still cousins or not (and Romeo/Benvolio) BUT Tybalt always knew he would (have to) dedicate his life to them either as a knight or some sort of right-hand man à la Hubert Fire Emblem? some position like that (supposedly Rosaline as well)
- their generations clash and antagonize each other as well (mainly meet at social gatherings, political visits and later their respective 'trainings')
- Mercutio's connection to the prince isn't known (except from the Montague adults i think) and to the other houses he always seemed either like a distant family member of the Montague or some youngster from a "friend"/ally family they let stay on their estate for Reasons [so he's high/higher status isn't known to, among others, Tybalt (and the other Capulets). i said only the Montague grown ups and in charge know but he could have decided to tell Romeo and co]
- Tybalt was already trying to pick fights with Romeo and Mercutio often (and respectively, for Romeo, I'd say) but when the Romeo/Juliet thing happens it gets worse [might add an incident with Rosaline as well, and Romeo not taking the rejection very well] [idk if it would have been more personal, to Tybalt, than just the regular Montague vs Capulet thing tho]
- potentially Tybalt/Mercutio "rivalry" at training classes, if they took the same (heirs wouldn't have been training there, even if they wanted to became knights on the side / get the skills of a knight)
- Tybalt (and Rosaline) could understand Juliet wanting to run away but elope??? you barely know them!!!
- also he wouldn't say but I doubt think Tybalt would take the whole "leaving them (tybalt and rosaline) to deal with all the consequences including one of them having to step forward and take the heir role when 1. they don't want it (either) (cos i doubt Juliet wanted it, who'd want it) 2. they have been preparing all their lives for a whole other role
- shit blew up, the fights between the houses escalate while they're also searching for the "lovers" (or maybe they found them/one of them and they like confined them or something)
- Juliet dies (gets killed, likely), not sure for Romeo
- Rosaline might have been killed in all the fighting as well. If they survive they either get forced to assume the role of heir or ran far away (with Tybalt's help)
- Mercutio and Tybalt aren't dead, duh. might have been more or less severely injured in the big fight, i don't think they really would have opposed/fight each other, they were more likely trying to protect Romeo and Juliet, respectively (tho Mercutio might have back off the whole thing bc "a plague on both your house")
- they might have fought/argued violently before the big stuff broke out but managed to realize they kind of wanted the same thing aka for their friend/relative not to do such a dumb thing but also if they're determinate to do it, gotta help them (Tybalt does not want to kill Romeo in this AU, but he probably wanted to convince them to give up their marriage and eloping idea) (and was pissed)
- except Romeo shows up and it becomes a mess. Maybe they misunderstand something too and that's why they start arguing with Tybalt and the other present Capulets, neither Mercutio or Tybalt can't get through them and they end up pissing off the Capulets too much (maybe ""encouraged"" by the other Montague who, too, are still hellbented on fighting and shit) and then some Capulet stabs Mercutio before anyone managed to reason with them > cue the "a plague on both your houses" (but he doesn't die of it)
- back to the aftermath of the big conflict: Capulet house is mourning (Montague too if Romeo didn't die) but both of them are basically already preparing for an all-out "war" (that would only concern the both of them but wr felt appropriate in strength and bloodlust)
- Mercutio (no matter if he backed off earlier or decide to do it now) gets called back by the prince (which might not be the PrinceTM of the country but just a higher noble supposed to supervise the other houses/territories) and officialise him as Lord
- may not have been given a territory (yet) but that's because he didn't want one? in any case he's tasked with quelling the fight between the Montague and Capulet and bringing both houses down if needed (either bc the hate is too strong / things won't be resolved even if the acting heads of the houses resign or get arrested + if investigations show the houses have been doing shady stuff) (idk if it's the case tho)
He got "given" a taskforce for that / allowed to recruit one
- while the Capulet were mourning, Tybalt (and Rosaline if they're still alive) got the fuck out of here : they both resent their house for 1. how they dealt with Juliet wanting to elope 2. not having been able to protect them 3. mindlessly conflict with the Montague and using the situation with Juliet (both before and after their death) as an excuse [basically their opinions of M and C are now that "a plague on both your houses" Mercutio line]
- if alive, Rosaline runs away , idk if to a far away territory or just to another country
- Tybalt on another hand, goes in hiding for a bit until he hears the news about Mercutio (and about his order/task, tho idk how public it is so tybalt might had to investigate for that) (or well get someone to investigate bc *he* wouldn't really be the most likely to get information) and shows up like "hello i'd like to help (i crave Revenge on both houses)". and gets officially dubbed a traitor by House Capulet when Mercutio agrees (tho they were already considering him, and Rosaline, traitors for fleeing and not supporting the house's actions)
- I imagine him with like four different blades pointed/against his throat when he comes meet Mercutio, I just liked this idea (bc 4 guards who either are very jumpy or recognized him (at least as a Capulet tho he wouldn't have been wearing their colors so either it's his face they knew or maybe he still had his old weapon with Capulet symbols on it wit him?) (he'd have dropped it before walking to Mercutio/as the guards surrounded him tho and only brought it as an identification tbh, he wouldn't want to keep it))
- and then after Tybalt proved himself enough, the Homage/that gay af ritual will take place as Mercutio will make him pledge allegiance
- at some point Tybalt's want of revenge will cool down (like he still want them to pay but it's less resentment and anger and more because they objectively need to pay and peace and order has to be brought back)
- he probably keeps talking/answering to Mercutio informally (from the start i mean) because that's how he's used to talk to him + he's still kinda of a fucker which earns him a Not really Good reputation at first + lectures from "superiors"/people "closer" to Mercutio
- It's probably not as bad as in canon tho because annoying/teasing him might be fun but here Tybalt has a real objective and wants Mercutio to let him stay by his side and trust him so he's aware he shouldn't push it too far
- not that he doesn't want Mercutio's trust in canon but yknow
- and uh that's it i think? apart from more kissing later I didn't thought of the rest of the "plot"
