Just to point out, that might be the entire point in the society of the Undead. If each group is trying to point out their own supremacy over the others, it makes sense why either no group would want to meet on equal footing (Perhaps outside of peace negotiations between guilds or groups or sects) and for things like a request of information, it would be a "Walk into my den to find out" as both a display of dominance and an attempt to intimidate your opposition. But if you want to say meet with friends, you could have a Den for your group. This way you all meet under equal footing, and it can double as a way to flaunt your group's dominance over other groups who wander in for either a discussion or for a possible attack.
I have a bit of an issue with this from a mechanical aspect, please bear with as this is a bit of mouthful since it touches on several principles.
If players should expect based on current game designs that they will party with other players, this propagates an in world factions system that is based on class rather than RP principles.
First, this never feels good. If you've ever played WoW you know that the boundaries for Horde VS Alliance always feels super artificial and stupid and if we're honest, kinda racist. Plus the plot frequently contradicts the existence of these factions being a long term staple as they often work together and then immediately drop all forms of negotiation and return to being at war as soon as it's convenient for the plot. All in, bad story telling, not that I think anyone is trying to push wow for a pulitzer and it is what it is, and serves it's function well enough, but I'd hope DHS aims higher.
This is because in DHS narrative is intended to be a strong point, where as with WoW narrative is more of a secondary background mechanic where largely your participation is virtually meaningless as the game world is identical and progresses as it does regardless of your action or inaction.
In DHS each class while being self sufficient is likely to have it's own strengths and weaknesses mechanically, and thus it makes sense to want to have a diverse party (whether there is holy trinity stuff or not). Having strict factions like that would either make that impossible or at least narratively discourage such things.
I don't think it's uncommon for people to gather in like kinds, as that's normal, but greater drama and story telling is generally easier to access when we don't rely on shallow characteristics like "you're vampire, I'm a vampire, we're friends now" and instead gets to the meatier heart of it when we consider things like values, principles and goals of a character. I don't think it's controversial to say that this is where RP takes the wheel and shines, not when you get a +1 bonus to your stat sheet.
Because players in theory "should" be encouraged to develop their own motivations, subtexts, goals and values, this means there will be a diverse cast and playstyles within the community in the game, hence why I think such a "neutral ground" is a necessity. I imagine courts will exist where say clans can meet and do political stuff at the local lord's sanctum, but again, not everyone is there (in DHS) for political intrigue and in many cases this can directly oppose a character archetype.
The one reason I don't like the marketplace is because there's every reason to shank your enemy while they are distracted and shopping. Pretty much all marketplace bazaar tropes center around these tropes of conflict, and probably for good reason if we dig in. Obviously tropes and expectations can be subverted, but that also requires some master strokes to make it believable.
I also agree that "something like a warframe relay" is exactly what I'm asking for, but would caution against using that particular design as a template as I feel they are one of the worst possible implementations of a player hub I have ever seen in a game, and I cite the fact that their implementation/participation is artificially induced and could just easily exist in any other context, likely to better effect.