The first 4 packs had unique skins but had old perks. After that, we have had homebase characters be in packs (which in my opinion is lame as fuck since the packs are $16 each). But I realized that there aren't many characters left that they can make packs with. The only characters left to my knowledge are Pop, Kevin, Dr. Vinderman, and Desiree. I highly doubt Pop or Kevin will get skins since they aren't as remembered well as Ray and Lok, while Desiree needs a completely new model since she has never had a 3d model to my knowledge and I highly doubt Epic has the "money" to make her a hero/skin. Same goes for Dr. Vinderman. Plus both are "dead" (we technically don't know since Epic can't add a basic Twine Story line to end the story).
At this rate, I think soon or later we will be getting the old stw packs back, starting with Metal Team Leader, and then it will be an endless cycle of old packs returning.
Source: https://www.reddit.com/r/FORTnITE/comments/piqped/is_epic_bound_to_run_out_of_characters_to_put_in/