Third-Party DLC was announced back in October 2017, at which time they were looking for people interested in releasing premium DLC. Now one and a half years later, we are still none the wiser what the new DLC's will be. In that time I have heard a lot of comments about the Third-Party DLC, but no one knew for certain what it is supposed to be. I have done some research and hope to explain to you what the new Third-Party DLC's mean.
WHO MAKES IT?
The most common misconception is about who makes the DLC's because this is not the first time there has been a Third-Party working on Premium DLC for Arma 3. The Jets DLC was the first and so far only Premium DLC made by a Third-Party, Bravo Zero One Studios. They were also responsible for the Encore Update, a free update that introduced the destroyer, USS Liberty. Bravo Zero One Studios was explicitly formed to create the Jets DLC, headed by the creator of the F/A-18X Black Wasp addon which won third place in the Make Arma Not War competition. So will make the next Third-Party DLC? We can't say for sure yet, but there is some information available.
In October of 2018, there was an Arma 3 Creator Bootcamp where 18 community creators were present, most of whom work on potential Third-Party DLC's. Some of the people attending were IceBreakr, maker of Lingor, Panthera, Isla Duala and many more maps, and LAxemann, creator of Dynasound, Enhanced Soundscape, several other mods and also the sound designer for some of RHS. All the Third-Party DLC's will be created by community creators, but what they make is not what they have already made before.
WHAT WILL BE MADE?
We don't know yet what will be made because the list of creators and what they will make has not been announced yet. But there are some certainties; the new DLC's will be unique content. Bohemia Interactive has said that they do not want the Third-Party DLC's to be something that was previously free. So we can only really speculate what could be made, could be something similar to IFA or RHS, or it could be smaller things like individual Firewill jets or maps. Who knows, maybe we will see a functional ship rather than the static ones we currently have, although I doubt it.
There is one big difference between all previous DLC's and the upcoming Third-Party Premium DLC's. The upcoming DLC's are optional installations! What this means is that the content is not downloaded to your computer until you buy the DLC, so if you don't own the DLC it will not show up anywhere in the launcher or the editor. Whether the DLC's will be used in our missions remains to be seen, at this point the complete functionality is unknown. Expect content that is up to Vanilla Arma quality, though. All the content creators working on Premium DLC have access to the guidelines for technical and quality requirement, get support with the Quality Assurance process and get support from BI for promotion.
Bohemia themselves won't be sitting still, however; they will continue working on improving already existing content, bring new content that builds on what already exists, and their Amsterdam office is working on something experimental that builds on Arma. In the future, we can probably expect more updates like Warlords and the NATO Pacific vehicle retextures but don't expect content that will drastically change how elements of the game work.
WHEN CAN WE EXPECT IT?
Simple answer, we don't know. There has been no information other than the original announcement and the updates of the Creator Bootcamp. We know that we can expect a few per year so that it won't be a very rare occurrence. When the DLC's stop coming is unknown, but the best guess is when Arma 4 is announced or is close to release; It is still unknown when that is though. I'm going to make some guesses here though; the first DLC will come later in the year, probably end Q3/start Q4, and I think Arma 4 will be announced in 2022 with a release in 2023, which will coincide with the 10th anniversary of Arma 3.
Again, all of this is speculation based on information already provided by Bohemia Interactive. I just hope this clears up some stuff for people.