With the source code and so many new private servers 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 going to bother writing about the existing popular servers because they are, well, popular

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 that might one day be just as good as nost, possibly better (please don't assault me for saying this, there is always a possibility).
1. Classic Warcraft http://classicwarcraft.com/
Has been under development for quite some time now, you can follow their progression on the webpage and they are active over their IRC (that Kiwi thing, first time I see this). 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.
2. 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, closed beta is coming to an end, open beta incoming... 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. They both seem to have dedicated staff that has been working for years, so I'm pretty sure both will be good once they are 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.
As you can see, I focused more on the upcoming servers because I thought they deserved it. The third option, CWS, could be a decent place to play at, I guess, but there's still some work to be done, advertising most of all. I listed it mainly cause it's a fresh start server that is open at the moment.
Classic Warcraft and Crestfall will very likely be quality, populated servers once they release, so I suggest everyone who wants a fresh start in vanilla to check them out.
Those are the first servers that come to mind, I'm aware that there are much more. Feel free to mention some that you consider worthy and keep in mind that I do not endorse any one of these. I found out about them a couple of days ago and now that we know for sure Nost won't endorse any one specific server, I believe everyone on this forum has the right to be informed about them.