It's weird huh, ofc there can be cross interaction, because basically on the alliance blizzard built every race to focus on humans, but if you thought about it more, actually they could fit in two camps, with some cross over
Main Alliance Group
- Humans
- Kul'tirans
- Gilneans (human aspect)
- Gnomes
- Dwarves
- Lightforged
- Mecha Gnomes
Guardian Group
- Night elves
- Draenei
- Worgen
- Void Elves
- Dark Irons
- Pandas
- Dracthyr
Night elves - b/c they are their own thing, 15,000 year old race that has it's own distinct developments, civilization, empire, focuses on Elune, the arcane and nature, with fel in very unique ways, mostly based in the broken siles and north and western kalimdor - a bit obvious.
Draenei - well, many of their ideals, level of advancements, views are in sync with night elves, they crash landed on their territory too, made first contact and friendship with them, and they're purple too that elves
Worgen - Obviously, greatly helped and saved by the night elves, while human and the human element very much involved with the human half, the worgen element can be much more ruthlessly and primal-ly explored in the night elf group to whom they owe their stability.
Void elves - They're purple, and fit in civilization well with night elves and draenei. They may seem human at first, and there is the high elf element, but although not explored yet, void elves have strong ties to night elven Highborne, great interest in Elune's dark side which is powerful void magic and have an opportunity to play a large role in helping night elven druids protect the Emerald dream from the void by their knowledge and use of it - this is a good opportunity, and they're purple. Draenei get on well with Thalassians, and Velen was interested in Alleria, not to mention you have the Auchenai priest who use void, so there is that side to explore. Whereas humans are generally anti- void using elves.
Pandas - Pandas are an ancient pre-sundering established race who actually do have an ancient interaction with the kaldorei empire, though none today remember it (it was long distant ancestors), however their nature, animal like appearance of bears and rural/civilised duality I think would find the Night elf/Draenei/WOrgen axis far more their thing and would be drawn more to them.
Dracthyr - Again, another ancient race from before the sundering For I just think humans are more snubby with none-human races, and this would make the dracthyr fit with the night elves more. They have the same ancient race vibe, and dragons and night elves get on well. Blues, Greens and Bronze have all heavily been involved with the night elves, and Reds too - Alexstrasza did create Nordrassil
Dark Irons - they are here because they are dark, like literally every race in this group. Dark irons feel a bit closer to their earthern and thus ancient roots, which I feel also ties them better with the ancients race.They'd get on well with Dracthyr and Void elves, and we can have a human - dwarf interplay with Worgen and Dark irons like the dark mirror of the actual humans and dwarves in the alliance group.
There will be interplay, like except the high elf side of the void elves to be heavily involved with humans, and Lightforged to be often around Draenei even though they are based with humans. Worgen off course are split, so there is interaction, but you could literally draw a line.
Also I reckon that if you tallied the number of people that play those races. It would actaully be roughly even between the two halves.
Now as for the horde halves, I don't think you can neatly separate them along such clear lines in a way that would have equal numbers of the playerbase, but theme and story wise we'd probably have:
Main Horde
- Orcs
- Trolls
- Tauren
- High Mountain
- Mag'har
- Zandalari
Eastern Horde
- Forsaken
- Blood elves
- Nightborne
- Goblins
- Dracthyr
- Vulpera
Off course here Eastern horde is going to have many choices. I had Goblins and Vulpera in that group because frankly it needed more races and they were closer in development and style to humans and elves. Main horde is obvious. Dracthyr would be on the side of the elves and undead humans off course having interacted with both races - Nightborne would remember dragons from their night elven empire days, and undead from when they were alive. Also Eastern Horde needed more races. Monster based wise, I'd have put Dracthyr in Main horde if not for the imbalance in the number of races on each half. However if you count playable population, sorry, I think Eastern horde would just need Forsaken, Blood elves and Nightborne and they'd still be more players with them than the main horde also including Goblins Dracthyr and Vulpera.
Capitals:
If they were to expand to be a proper sub-factions then.
Main Alliance = Stormwind
Guardians = new city best placed in Kalimdor - thinking re-built Eldre'thalas with Draenei, Panda and void thalassian influences.
Main horde = Ogrimmar
Eastern Horde = Lordaeron or Silvermoon (my bet would be Lordareon unless blizzard has alliance affiliation plans for it, then it would or should be Silvermoon, unless they have alliance affiliation plans for that too, then Stratholme.
Other options for the sub-factions include Shattrath, Dalaran and Suramar - but those are cities while dominated by one race have both faction peoples heavily involved and no real reason to change that.