All of the older stuff really need something fresh. Plus they were developed without the core of the standard pack's identity being developed. These include GTW, DO, OR, etc. These early ones have bugs that need fixing, content that needs rounding out, and a bunch of general improvements.
The rest are more of a wishlist, for me, because they are the packs that felt especially deficient.
First, Kid's Room Stuff. Voidcritters have become recurring characters in the worlds, but their initial and most substantial appearance is far too limiting. They are only relegated to the child life stage. Teens and adults should be able to play with children on this.
ROM is another personal disappointment. It is not the worst pack by a long shot, but it lacks the punch and identity that Vampires had. We need unique abilities not found in other packs and more crosspack that make spellcasters feel more powerful. Also ways to make them more unique.
Cats and Dogs is the pack I have always been most disappointed in. I have expressed it in long posts in many other places, but Pets packs have been my make it or break it packs since the beginning and 4 was the only one to break it. In comparison, 3's was near perfection. So yeah, anything to make it more like that. Develop pets properly. I want control, but even without it they should have proper relationships and an info panel. Children should be able to do care actions like bathing and walking. Cats should be able to learn tricks. They should also have skills again and maybe be able to get jobs. They need more traits. Also, I would willing accept small pets, farm animals, and equestrian things in other packs.
IL, I love, but it could use more underwater and mermaid stuff.
In Seasons, it is a super solid pack, but I just want to be able to control the seasons! Had I that power I already would have had my own winter world ten times over.
GF's world could use some fixing up. It always feels so small an unusable whenever I play, and it is supposed to be a city but not a CL city.
EL I enjoy, and I do not mind NAPs too much, but I am not opposed to limiting and restructuring some of the more annoying behaviours. Plus things still happen where the inspections are a fraud. Plus more options are always appreciated.
Parenthood is another solid pack, but it seems to be the best place to fit school reformations. I'd like options like private school, boarding school, and homeschooling. Plus it also seems like the best place to tack on children's activities like treehouses, lemonade stands, etc. If not put here, they can come in other packs.