In the roadmap there's a chapter that gets a cosmetic pack, champion pack and a weapon pack while other chapters get only a champion pack. So what determines which chapter gets everything or just the champion pack? 

Likely just decided based on what chapters are popular and which ones have iconic details on the marines and/or champions.  You can look on GW’s store at the chapter conversion packs and unique units, and get an idea of which chapters will likely get what.

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orwhynotrandom wrote:

In the roadmap there's a chapter that gets a cosmetic pack, champion pack and a weapon pack while other chapters get only a champion pack. So what determines which chapter gets everything or just the champion pack? 

It will most likely follow the more popular chapters. Those being Blood Angels and Space Wolves and maybe Black templars as the most popular chapters, so they will receive the champion pack, skin pack and weapon pack. Then the other founding chapters receiving just a champion pack. with probably Salamanders and Imperial Fists first followed by the remainder in due time.

​Bonjour. Qu’entendez-vous par « pack de champion » ? , l’ensemble des pièces d’armure et de l’arme d’un champion de chapitre ?

I’m pretty sure they said Dark Angels is the next chapter to get cosmetics. 

You can also propose a cosmetic for the chapter that you like on ideas. For example the Blood Ravens have had a very big push!

Here it is as an example, dontforget to give it an upvote and follow. The more it has the more chances it has to catch the attention of the devs;

https://community.focus-entmt.com/focus-entertainment/space-marine-2/ideas/372-blood-ravens-20th-anniversary-cosmetics?page=1

I hope it helps.