So now that I have a bit more time on my hand I can give you some proper info for your game.I won't focus too much on the leaders of the factions,as Epicurus did a good job with that,so instead I'll get more into detail about what I think the different playstyles would be like:
Demacia
Demacia would be kind of an antimage and elitist faction.Their arsenal of special abilities would likely be rather limited as they usually don't use mages outside of a secret order,but they'd have abilities to cancel out enemy ones.Since Demacia is known to have one of the best trained and most organized military,so I'd expect their units to be stronger than most average units and that their units would synergize well with each other.
It is also a land that is known for its abundance of resources,so while Demacian units would likely be a bit more expensive,they'd also have more ways to pay these costs.Aside from their regular soldiers they are also known to have a flying cavalry,I think it was said that they are black wyvern riders,though I remember that there was also a picture of a guy riding a giant bird.
In terms of leaders I'd say that J4 could focus on the synergy between units,while Garen could focus on the anti-magic aspect of them(or Galio focuses on the anti-magic aspect and Garen on the training of the units)
Noxus
Noxus simply uses every tactic to their advantage.In a game they'd likely do a bit of everything,but would likely rarely have effects that synergize with others.They do use warbeasts,like the drakehound or basilisks,but also have mages(including blood mages),powerful warriors and assassins.
As for the leaders I suppose that Swain could be more about magical power and gathering information from the enemy,Darius would be about raw power and Lb(who is likely the faceless and is the leader of the black rose eitherway) would be about tricky effects and maybe assassins.
Piltover
Uses a lot of hextech technology which they'd likely use in a war scenario,their average soliders would likely be equipped with powerful gear that would put them above your regular foot soldier,even if they aren't that well trained.With a vast array of hextech devices I'd expect them to come with lots of different abilities.
As for leaders I'd suggest Caitlyn,since she is the leader of the police force and Camille,who is the shadow leader of one of the biggest clans in Piltover.
Zaun
Is similar to Piltover,but usually uses more crazy and unpredictable hextech gadgets and would likely use more augmented soldiers,while also using poisonous chemicals to fight the enemy.
Shurima
Shurima honestly doesn't seem to have much of a military right now except for Azirs sand soldiers and the mind controlled servants of Xerath.In the past though it looks like they used generally tried to overwhelm their enemy forces with a bunch of slaves,with some really powerful ascended as the backbone of the army,though nowadays they are pretty much rebuilding their armies,so it is difficult to say how they'd work now.
Ionia
Is known as a land of magic,so they'd likely focus a lot on magical abilities.From what we see in the game,it looks like their units were usually focused on having low defense in favor for a high offense,yet their nation is not an agressive one.Recently they also started to work on their own hextech devices,as can be seen with Jhin,but otherwise we don't know much about their modern warfare(I guess we'll learn more about that soon,since they announced a small ionia event).I guess in terms of leaders one could focus on Irelia for the militar,Karma for the more spiritual aspects and Shen/Zed for the ninjas(with Shen likely focusing more on punishing enemy agression,while Zed would focus on destroying the enemy to gain more strenght)
Freljord
Not much here that I can add that Epicurus hasn't said already.
Though one could say that the Avarosans would likely try to overwhelm enemies with their numbers(as the avarosans are known to be the biggest tribe) and the frostguard would likely use some powerful ice magic to their advantage.
Bilgewater
Likely the strongest faction when it comes to Sea battles.
Currently it is mostly ruled by Miss fortune(as she killed Gp and most other big leaders),they have a lot of gun powder based weaponry and there are also so called serpent callers,who are able to control giant sea serpents.They are also closely related to the Buhru islands,who are likely a bit more tribal.Besides MF,one could also use Gangplank and Illaoi as leaders,with Gp being the former leader and Illaoi being the figurehead of her faith.
Shadow isles
They attack without much of a warning and their undeads are immune to most weapons(though demacian steel can hurt them).They also essentially turn every fallen enemy into one of their own.
Targon
Since the mountain seems to be infested with void horrors,I'd say that the average targonian warrior is quite tough and brutal,however they are low in numbers and don't seem to have many special warriors except for the aspects.
The void
Not much of a faction,as it is more of an origin for several creatures.
The void is all about erasing things like biomass or even memories and it produces monstrosities that start out rather formless originally,but then turn into more solid monsters eventually.
It does need to consume living things in order to spread though,so if it isn't able to devour anything,it will stop to grow.
While it obviously doesn't have leaders one,could use Malzahar as the leader of the void cults or Cho'gath who seems to be the most powerful voidborn champ in the game and represents the whole consuming aspect of the void better than anyone else.
Bandle City
Sadly there isn't really much we can say about it.
However judging by Tristana and Teemo,they seem to favor ambush tactics in their warfare and the new lore also makes it clear that it seems to be hidden from most regular people,so it would probably be good at tricking the enemy into a false sense of security
,because LB as the master of "guile" makes a lot of sense and it was hinted that the faceless is a woman and that Swain is willing to use the black rose for his own agenda,even if the black rose wants him dead.