The 5 most important things that you need NOW !

There is more than this, but the top 5 would be:

1. Expanded roof customization and proper attic spaces

2. Spiral stairs

3. 'ConstrainFloorElevation' cheat

4. No terrain over basements / Proper walkout basements / Basements with windows

5. Invisible foundations / Floating floor tiles

At the minute I have TS4 on two devices, one with a generous amount of packs and one base game so I'll split it up.

Pack Related

1. More pack refreshes, I bought and loved Spa day and would love a reason to justify the bowling or movie night packs but they're too primitive to be worth the money.
2. More activities for younger and old sims. There are a lot of hobbies that can be done by adults, but I would love to see some more hobbies for older people (like maybe bingo, scrapbooking with the ability to add pictures) and things for kids to do. Softplay centers, sometimes heading home with crafts from school.
3. More cutesy build buy. I want the pastels, I want the retro, but please stop putting them in only kits. I'd rather spend a bit more money and get enough things to furnish homes in lots of ways. Like the b/b from Tiny Living is incredible, and versatile enough it mixes with other packs, but cozy kitsch is stunning but basically only matches with stuff from different kits, and I am so firm on my stance that I will not pay a fiver per room in a sims house, I will simply stop playing the game before that point.
4. More retro clothing kits. I think CAS is super versatile with kits as clothing is a lot more personal and specific from player to player. I'd love a 70s kit, a 90s chic kit, a really cutesy style kit for clothes. Just things that allow a bit more storytelling of personality.
5. More 10 level skills. I keep thinking how well thought out the author and tech base game careers are, and it's not because of a weird bar you're fighting to keep level, it's because they're interesting to play several times over. More of that please. 

Base Game (aka things I can show my friends to get them hooked and make EA more money when they inevitably become customers like me)

1. Scenario prizes. I've seen a lot of complaints about the battlepass style events recently, and I have found a solution to most of them. Instead of shallow and easy to miss events, specific scenarios with items as rewards could be implemented as well. The scenarios could have three parts, with three decent rewards and could still be themed and promotional in nature. A bonus is that to unlock the enticing items, the player is likely to be influenced to get the packs long term, not just release week. The player doesn't miss out on extra content if they're willing to put in the work and it's lasting promotion.
2. More kitchen counters. It took me hours to build the kitchen, because I could not manage to make anything work the way I wanted. I thought my furnishing had improved with the years I played, but it was just the improvement in quantities of kitchen stuff I got from packs.
3. Bed swatches. I felt cruel giving the fake children beds that looked fit for a prison, but it didn't make sense to splurge on the space looking one that didn't match their personalities. It's a little less grim with double beds, but still needed.
4. Ways to earn simoleons. I love having the sims create plugins, garden, paint, take photos and write books, but there could be a few more pathways for sims to produce items in the base game, particularly ones that are a bit more modern than those mentioned above.
5. Roof shingles and wall exteriors. It was painful trying every combination to try and find one that I didn't dislike. I still don't like the ones I've been using, but the textures and brightness are intense and very 2000s so they are the best of a very bad lot.

Hope this wasn't too long.

1. Active careers for Chefs/Mixologists/Musicians/Comedians. Every work day takes your Sim to a new location. Chefs can work at restaurants and be hired as the caterer for social events. Same with the mixologist - bars, social events, and restaurants that have a bar. Comedians and musicians can perform at any community lot that has a mic/instrument, and also social events that require those services. For social events, all these career paths can even contribute to whether or not the host receives a gold badge.
2. More empty worlds like Newcrest. 
3. A pack focused solely on mixology - appliances that create base ingredients for drink making like stills, fermenters, etc. and a juicer for harvestables. Maybe even a drying rack for botanicals? This would tie in nicely with the simple living lot challenge where all these ingredients will be needed, and for the active mixologist career mentioned above. 
4. More Spellcaster interactions that involve pets and infant-child Sims. 
5. A skill tree for Mersims and Aliens.