With the source code and so many new private server appearing every day, I'm pretty sure our best options for vanilla are:
a) Existing servers like Kronos, The Rebirth etc. Kronos is already swarmed by nost people, rebirth less so. I guess they are both good enough servers, though I've heard some nasty stuff about rebirth staff being corrupted and whatnot. I'm not gonna bother writing about those two because most of those stuff are general knowledge and I wouldn't really have anything new to offer.
b) Check on the servers that have been under work for quite some time now, which means they aren't just some vulture server trying to profit on the downfall of nost but actual, legit places.
Classic Warcraft
http://classicwarcraft.com/ has been under development for quite some time now, you can follow their progression and they are active over their IRC (that Kiwi thing, first time I saw it). They seem devoted to the idea of making a really, and I mean REALLY high quality server, which could take some time, but once they do release, I think it will be one very nice server.
Crestfall
https://www.reddit.com/r/CrestfallServer/https://www.reddit.com/r/wowservers/com ... c_setting/You can see that they have been working on this for some time now, they will have a webpage on very soon, open beta... They have their youtube channel with some videos, promising more.
These two have gotten my attention because once they are released, they will be fresh start realms, won't offer nost players any transfer, and in terms of progression should be rather close to what nost was when it released.
c)There is one server going on right now, Classic WoW Server
https://classic-wow-server.net/. Honestly, it doesn't look like much, but I've decided to stick around for a couple of days, see how it goes. They claim they've been working on it for over a year, and when Nost went down, decided to release a bit before they had originally planned. However, they did a shit job with advertising (none, to be correct), so even though right now they are a fresh start vanilla server with a decent quality, they have almost no players. Imo it is worth at least checking out while we're waiting for other options.
Those are the first servers that come to mind, I'm aware that there are much more.