Depends on the time frame of the Game whether the Soul Drinker are dead, loyal, or mutants really. I don't care enough about the Eldar to know if they have those kinda factions but I'll take a punt er...the pathfinders and pirates that don't walk a chosen "path".
Orkz, Again I don't know if they have a faction like those I mentioned above. Might need an ork fan for that

When I asked if you thought they'd make an appearance I had taken it as a given that they wouldn't be playable as we have already had a playable races list and they were not on it, I meant more as NPC support or "quest" givers if there's that kinda content and its not just a Planetside 2 "nothing to do but fight all the time" theme. I kinda hope there's some of that singleplayer content. I don't wanna have to grind but some story missions would be welcomed

A quest line to try and discover the origins or location of the Legion of the Damned would be cool for example. Or Grey Knights turning up and bein all like "yo, go punch some chaos marines in the face til they die and we'll give you this uber relic"

Ugh this part-

"When I asked if you thought they'd make an appearance I had taken it as a given that they wouldn't be playable as we have already had a playable races list and they were not on it, I meant more as NPC support or "quest" givers"

sounded way more arsey than it was intended to be, wasn't trying to sound like a dick

The thing is though that they have already said that the only role (friendly) NPCs will have (unless I've really misread the different articles and interviews) was potentially to be guarding bases. Do we really want such a bad ass chapter to become guards? It's neither lore-friendly nor would it look good. I can imagine a couple of years down the line that players themselves can spawn as one as some sort of special unit, but I doubt you'll see one guarding bases as an NPC or have a big, yellow questmark above their head.

they could be a power of the war council to help in dire situation

for orks it would could be pirates

There are less than 200 LotD members still alive as survivors of the Fire Hawks Chapters, too lore breaking to include.

Grey Knights rape absolutely fucking everything apart from Greater Demons and Daemon Primarchs (they have to lose marines to deal with these). Too lore breaking to include.

Smaller chaos warbands? I don't see the reasoning over using legions.

Deathwatch, Can't see the reason to include the deathwatch as a separate sub-faction since they are a small elite force. They don't operate as a whole ever.

Game breaking you could argue, if they were playable
I don't think you can claim they would break the lore when they are canon

I h=guess it may not work if there's no pve elements out in the world but even in instances you could use them for plot

They are canon but to use them in gameplay breaks the lore, it defeats the entire purpose of the Legion of the Damned to allow mass amounts of players to play them. Even more so than normal chapters.

Same for Grey Knights they are never supposed to be deployed en mass unless it's something like an extremely large daemonic incursion.

Deathwatch never operate as a full force they operate in teams.