Well, for the record, I would support somewhere else (Barovia/Port) as a second RP hub, but if you insist on the Mist Camp, it would be fairly easy to "flesh it out," since, as you say, it isn't canon and can be defined by us.
Its a camp. Its moveable. It could literally be anywhere, within the Mists themselves, or on some small pocket of land encapsulated by the Mists. Use this. Make the area roughly 8x, or more, the size it is now. An island of land in the Mists themselves, with a semi-permanent Vistani Camp in the corner of the map. In the REST of the map, make a ruined village/town. Somewhere that was swallowed by the Mists and forgotten from memory. Now that you have that, with buildings (and possibly ruined buildings, crypts, cellars, sewers, etc. to explore), populate the area SPARSELY with what essentially amounts to "entrepeneurs," in the form of people (Outlanders, Core Realm Expatriates, occasional Criminals) who have decided to call the place "home," and "set up shop," as it were. An Inn, a Bar or two, perhaps some people who make useful things, etc. MOST of the town will be utter ruins (probably haunted by the original residents), but a small section, a few buildings, will be renovated/fixed and used as businesses. Adventurers are rich...everyone knows this (even if it isn't true), so why not make some coin by having a business there? Sure, spirits may roam the streets at night, and its a dangerous place to live and work...but so is the REST of Barovia/the Core. And it may be LESS dangerous for some of the people who live there than where they came from (criminals, traitors, etc.).
It could still be dangerous as hell to wander alone at night. Just like the Outskirts, Vallaki, Port, etc. The ghosts are unfriendly. You could have some basic crafting stations, possibly even ruined ones, or you could just make people go somewhere like Port to craft...its a 3-minute, 100-fang ride. Or, better yet, put some things there (you decide what), and ONLY there, to give people a reason to be there, and hang out. Perhaps some specialty shops that sell things people actually want/need, an NPC that hands out quests for new areas in Port or Barovia (or anywhere else), etc.
But, as it stands right now, using the current Mist Camp as a hub would be completely wrong, IMO. The Vistani would pack up and move if that happened.