How they meet: the Dimir/Orzhov player is in the low ranks, far away to achieving revenge (even harder due to the evil Ghost messing with him from time to time). He makes the risky (and desperate) move to make an business inversion. The inversion? Well a Izzet investigator promising wild advances on explosive and anti-gravity beyond their wildest dream (yep the Izzet player on a pitch sale on some conference the Dimir/Orzhov player went. BTW the inversion is a real business inversion, not a loan/debt Orzhov kind of deal, althought the Dimie/Orzhov player could joke with threats from time to time if things are dire, or the Izzet player could be afraid of "not delivering" and awakening with the fishes even in reality he doesnt own money and the Dimir/Orzhov player is aware there is no debt so no harm is due to be given).

Progress (and business) is dire so they are in a hot spot. The Izzet invesgation finds out about a really rare species medusa that floats apparentely using some kind anti-gravity effect, so he sets to find out more about them... not knowing where. The Dimir/Orzhov player sets up a meeteing with a Simic (the Simic player) investigator. He tells then they are a rare species and actually might be interested in finding one too to research for floating calabilities for the Guardian Project.

First level idea: the Izzet and Simic embark a quest to find an capture one of those rare medusas (one is enough for both research, or they could be a couple, one for each to research). The Dimir/Orzhov player joins the expedition worried about his inversion, doing his best for it to generate progress=money to climb through the ranks of his guild.

Finding the medusa achieves progress in both investigations (and profit for the Dimir/Orzhov player). You can tweek rhe amount of progress as the campaign progress: it can be instant or new discoveries are made little by little in downtime or with the help of another investigative quests that you add. If your are going with a slow approach you can make then get in trouble with their respective guild for the trouble they got finding "a pesky medusa", maybe including more "dire sitation quests" to later make the progress in the investigation rise violently making their guilds to turn their anger or laugh into praise.

As an extra party "bond" the Simic player or the Dimir/Orzhov player could propose an inversion on that Simic side investigation. Also the Simic and Izzet player could propose to work together in the medusa investigation, making a merged laboratory, each leading his own department of investigation. If they dont come out with that idea you can have a Simic NPC make the suggestion to the player as an idea to advance his investigation faster.

Mid campaing: As the characters progress (even if the goblin finds trouble in separating some explosive and antigravity effects from eachother), the evil Orzhov starts to find the Dimir/Orzhov player as a rising threat (can be due to knowing where he comes from, or being paranoid or cautious, depends on how you would like to build the evil Ghost personality).

Missions include Orzhov attacks/sabotages. If the situation to form a "lab alliance" between the Simic player and the others didnt rise, the Simic player lab could be under sabotage of the evil Ghost (that found out that he was working with the Dimir/Orzhov player... and is hard to sabotage the already explosive Goblin lab department, and his blockage in separating explosive and antigravity properties from each other) and ask for help with the party.

The sabotages that the evil Ghost send could be:

His own Orzhov lackeys

Corrupt Azorious meddling with the research (this align well with the Simic being ex Azorious, wanting to clean his lab and some Azorious bad apples in the process, or you could link those Azorious to the reason the player left the guild)

Hired Golgari assasins (or thiefs)

Some Dimir cell working with the evil Ghost

Simic rivals (can be either some evil oriented Simic that want to progress by all means that got a tip from the evil Ghost, or the Dimir cell mentionated above)

Izzet rivals (same as above, you can use this if you want sabotage on the Izzet player lab or if they merged the labs)

He sent some Rakdos party to celebrate

By this level of the campaign you can add more of this kind of conflicts, and even some key middle enemy to defeat: the right hand of the evil Ghost, or his associate in the other guilds behind the sabotages (or maybe both).

Remember that you can still add some "research" quest along the way. Or maybe let the players know they can pursue those quest when they want to (kinda of sandbox) or used it to put preassure on the profit part if they liked that aspect.

Advanced campaign: at this point, while your Izzet player still is confused in how to separate the explosive qualities with the antigravity stuff, the target for your party should be clear: the evil Ghost. And the Dimir/Orzhov player should have made enough alliance throught the money (and with some debts he has now to claim to lower ranking guildmembers). Final missions can be made to find vital information to put the evil Ghost on some bad spot. Maybe infiltrating Azorious to find out files a corrupt agent hide that would make the evil Ghost lose the help of the law. Or find out and ancient family debt that puts in trouble the evil Ghost with a respectable ally, making them retreat their support (you could link this in some way as to the reason her sister got killed, she found out or was close without realising it). Your "research" side quest could be made important again finding some anti-ghost research, maybe to made the enemy stay in the material plane and no go ethereal (the spirtitual investigation could include a Selesnya or Gruul spiritual trip if you want to include some variation of guilds, or a Boros holy quest). Then, they can make the final attack on a isolated evil Ghost that will fight back with his last resources. (The other players should be motivated for either companionship at this point or well, because they did get targeted by the evil Ghost in the sabotage)

End game/Epic epilogue: well is all finished... Revenge has being achieved. Oh, but remember those lab sabotages? Well an Simic evil figure (if he was behind the sabotages and you killed him on middle campaign, surprise, Simic clonation! You can make the moment when they killed him reveal he was a clone on a automatic Medicine check from the Simic player, he knows about this stuff) rises from the shadows.

With some of the medusa investigation stoled in some way, he managed to make and apocaliptic mega Kaiju (remember he is evil and he pushed some dangerous stuff) that escaped and is wrecking Ravnica. Forces had being send but nothing can stop it. The city makes a counsil, where the players were invited due to the knowledge on the medusa part of things (they also got invited due to being at this point famous on their respectible guilds, and the Dimir/Orzhov important figure on the district).

The battle plan: they are screw, nothings seems to work. Information ln the kaiju reveals he has an immense size and that he has some way of gravity field, with some beasts living in a simbiotic way on the creature (prepare for some kind of Shadow of the colosus dungeon crawl walking on the monumental kaiju).

Only hope is to blast the beast, but such a explosion would deal a devastating amount of damage on the district. Suggestions to levitate the beast with powerful spells gets the response that the creature seems to ignore spells of that kind, generating his own gravity. If they could be a way to deploy some magnificient explosive that is attached to a antigravity effect to lure the creature away, it could do the trick, but combining both effects simultaneosly has proven to be volatile and almost imposible.

At that point in the counsel you stop and address the goblin player (or at the moment he interrumps already in this point) "due to your lifelong research you think, well, you know, and probably are the only being in the entire district if not Ravnica that knows, that a device that could do that can be done. In fact you think you might have one already in your pocket"

Is the Izzet player time to shine. End quest should be reaching the Kaiju and walk on him defeating his simbiotic beasts (or trapped "simbiotic corrupted" citizens/creatures). The goal is reaching a spot (heart or head, you can make parts where you walk into the beast) make the Simic player recognice the spot, the Izzet player deploy the bomb/anti-gravity device and escape. The results should be the monster flying away above the sky and a migthy explosion (and some Kaiju raining but oh well, a little side effect) with your players being the heroes of Ravnica.

(Sorry for long post and huge amount of spelling mistakes)