Rainbow Six Siege is still going strong, strong enough that Ubisoft is still looking to further expand the universe with their Rainbow Six Quarantine spinoff. Though the game has been pretty heavily delayed and very little concrete info has been released, it appears that it's going to be influenced by the Outbreak event that Siege saw quite a while ago. That update featured the release of two new operators, Finka and the controversial Lion, as well as a robust PvE mode featuring monsters.
One thing that's all but certain in Quarantine is the inclusion of at least some of the operators from Siege. Maybe not every operator is going to be included, but many operators from Siege have gadgets and playstyles that should fit really well with the kind of game that Quarantine appears that it's going to be.
Attackers
While the Attackers and Defenders in Quarantine are probably going to be working together as opposed to murdering each other, they're going to split up here for the sake of organization. These Siege Attackers should probably be included in Quarantine for a variety of reasons, while the ones that weren't included have gadgets that might not be as useful when fighting a horde of monsters.
Ash - Ash is an obvious good choice here; her gadget allows her to create easy soft destruction, opening up walls, doors, and windows with the utmost ease. Depending on the health of the enemies in Quarantine, it might be good for more than just soft destruction, too; she might be able to turn some monsters into little bits with her breaching round.
Sledge - Sledge is a simple man, and that means he can be useful in just about any scenario. He brings a big hammer, and with that big hammer, he can make some big holes for the team to squeeze through.
Montagne - Montagne's got a big shield, and that means any enemy that fires anything at him or his team, he'll be able to block it. While most of the monsters probably won't be firing anything, it's likely some of them will, so Montagne could be of massive benefit in this regard, or in just aggro-ing the beefier enemies.
Glaz - Glaz's weapon hits hard, and he can see through smoke with his thermal optic. He should be able to post up from a decent distance away, or blanket an area with smoke to confuse the monsters and deal with them with ease.
Fuze - Fuze's ability to fully clear a room without entering it can be invaluable, and it will be especially so when there are more than five enemies.
Blitz - Much like Montagne will be helpful, Blitz could be very useful, but in a slightly more aggressive capacity given that Blitz can sprint with his shield and blind enemies.
Buck - Buck is a slightly more versatile Sledge, though he can't quite manage the amount of soft destruction the hammer-man can. Still, he's very useful, and that underbarrel shotgun could be invaluable when monsters are rushing him.
Capitao - Capitao is slightly undervalued in regular Siege, and it remains to be seen whether he will still be if he's included in Quarantine. Regardless, his incendiary bolts could be great for clearing a group of monsters, and his smoke bolts could be great when used in conjunction with somebody like Glaz.
Ying - Ying has a better capability to blind opposing characters than any other character in the game, and when facing a room full of monsters, this should be highly beneficial.
Zofia - Much like Ash, Sledge, and Buck, Zofia is capable of creating quite a bit of destruction. But, Zofia comes with the added benefit of her concussion grenades, capable of stunning enemies for a short while, too. (Plus, she would fit in with the monster scene in the new Zofia Resident Evil Elite Set.)
Lion - Lion released with the original Outbreak update for Siege, so it only fits he'd be included in Quarantine, too. His ability to temporarily mark any moving enemy could be great when there are dozens of enemies coming at the same time.
Finka - The other operator that released with Outbreak, Finka is capable of giving all of her teammates a temporary health boost and revive them from a downed state, as well as get rid of any negative effects they happen to be suffering. While it remains to be seen what negative effects these monsters will be able to inflict, if any, it'll certainly help Finka's friends avoid that negative effect of being dead.
Nomad - Nomad fires an Airjab that detonates when enemies get nearby, launching them in the opposite direction and possibly through a soft wall. When enemies are coming from multiple directions, that could be of massive benefit.
Gridlock - Gridlock performs a similar function to Nomad, though her gadget slows enemies down as opposed to launching them. Still, this could be just as useful, if a little less entertaining.
Iana - While it might seem like Iana wouldn't be all that useful in a situation where her team is surrounded by monsters, the possibility of her decoy distracting and leading monsters on a wild Iana chase has some real potential.
Flores - The newest operator added into Siege, Flores has some interesting possibilities. His explosive drone gives him the opportunity to blow open walls or blow up monsters without getting too close to the fray.
Defenders
There a lot of Siege Defenders with something to add in Quarantine, with many of them capable of fortifying an area with some serious hardware.
Tachanka - Complete with what is essentially a mobile turret and an incendiary grenade launcher, Tachanka's proclivity towards destruction is unmatched by any.
Oryx - The Kool-Aid man of the Siege universe, Oryx has little use for doorways and instead busts through walls himself. He could theoretically knock down a group of monsters at the same time, or open up new routes for him and his team to go through.
Smoke - Smoke has an innate ability to deal damage to a room full of enemies with his gas grenades, which would likely come in very handy when that room is full of things that want to kill him.
Kapkan - Kapkan can set up five traps at a time, both giving him and his team a good idea of where the enemies are coming from, as well as dealing some serious damage to them while they're on their way.
Castle - Castle can make a position much harder for enemies to make their way into, which could mean it's possible to funnel the incoming monsters into a more narrow area, making the position more defensible.
Goyo - Goyo's deployable shield cover might not seem too valuable at first glance against enemies without guns, but considering the fact that these shields explode, they could still prove very useful.
Pulse - If his team isn't sure where the monsters are lurking, Pulse has them covered. That is, if those enemies have heartbeats. If they don't, it might be best to leave Pulse at home.
Doc - Doc is great in just about any situation, given that his gadget allows him to heal both himself and teammates. If a teammate is having a rough go of things, give them a shot with the stim, and they'll be back on their feet in no time.
Rook - Much like Doc, Rook is great in just about any situation, too. His armor reduces incoming damage and increases the bleed-out timer for anyone wearing it, making it a good way to ensure a bad situation doesn't get worse.
Valkyrie - While surveillance doesn't necessarily seem like it might be too valuable in Quarantine, it's possible certain situations are going to call for getting a heads up when and where enemies are on their way. Valkyrie is one of the best at accomplishing this with her Black Eye cameras.
Mira - With a group of monsters that might not be able to attack through a small window, Mira could put her Black Mirrors in areas that give her and her teammate's easy shots on the enemies without them being able to attack back.
Maestro - Similar in idea to Valkyrie, Maestro should be able to get an idea of where the enemy is rushing from before they get there. His cameras come with the added bonus of being able to fire lasers at the enemies, though he doesn't have as many cameras as Valkyrie, and he can't be as versatile with where he places them.
Alibi - Much in the same vein as Iana, Alibi could get monsters to rush areas that aren't going to prove fruitful for them. Whether used as a distraction or just as a way to slow them down, her holograms could prove useful.
Clash - When it comes to Quarantine, Clash would likely be a more useful version of Montagne. She too has a full body shield, though she can fire electricity from hers, and she can come equipped with an SMG instead of a pistol or a revolver.
Melusi - Melusi's Banshee gadget doesn't make a whole lot of sound, but it does considerably slow down enemies who are on the way. This could be a great way for making a path where a lot of monsters are streaming through a bit more manageable.
Rainbow Six: Parasite is in development for PC, PlayStation 4, PlayStation 5, Xbox One, and Xbox Series X.