I'll echo a sentiment... story progression. Unless you plan to have one kid and only one kid per generation, a lot gets full quickly when three kids grow up, marry and stay in the same house. That leaves only two slots for kids. Moving them out just means they do nothing but age. They don't go to work, they don't earn money, they don't fall in love/get married/have kids without having to physically play it myself... or use MC Command Center. But even that mod doesn't quite work the way I'd prefer, but beggars cannot be choosers.
We need more personality development. Three slots is good, but unless you complete an aspiration with a bonus trait, buy one with aspiration points, or cheat to add one, you kind of get to a point where all the sims feel kind of the same. Yeah some are kleptomaniacs, some are romantic, but they all feel like they're the same person with a slightly different buff depending on the time of day/what they just did. The parenthood personality stuff is nice, but I'd love to see expanded personality traits and trait slots for more unique sim creating.
A deeper game. I've said this a few times in a few places recently, but there needs to be more depth to the game. I feel like the only real motives in the game are to grow up, have kids, make money, and keep doing that until you get bored and create a new sim to do it all over again. I just also feel that the aspirations need more depth. I can knock off several of them in a sim's lifetime. If this is someone's life goal, they're not all that easy to attain. And in TS4 it feels like it was made needlessly easier than it every should've been to attain these life goals. And the work from home feature isn't really working from home. It's basically do a chore and get paid. Expanding the game laterally is great, but deepening the game would be best in my humble opinion.
Insecticide. TS4 is really, really buggy. I don't know if this is because so many packs and updates cause it to be so... buggy... but by TS5 this has to kind of be worked out. I still have to save and exit and relaunch a save to stop my sim from changing back into her hospital gown after giving birth. People are walking upstairs past four sinks to wash a dish. Why do the sims have to play musical chairs when they have a conversation? Lots of bugs and kinks to be worked out of a game that's 5 years old already. I assume that means not much will come on that front, but with TS5, you have the opportunity to launch a game that's not nearly as buggy. Though, sadly, I've come to expect that from EA titles over my years of playing EA's sports franchises.
Swatches. Do you know how hard it is to coordinate colors in TS4? I know you've done a lot to make it better, but shading is a big problem. My sim has a dark blue shirt, but the pair of pants that would be best for it only has light blue or something to that effect. That fridge and stove go great together, but one is a mid-gray, the other is a dark or light gray. So, it's mismatched now. Eclectic some call it, but I think it could be better. I know it's a pain in the plumbob to give every item in every conceivable shade and color, so how about a nice paint tool so we can color match ourselves?
CAS. I don't know the programming that goes into a game like this, but I am certain that it's massive and complex. I do believe the average computer these days is leaps and bounds past anything that was new in 2015. Since many players of TS4 do seem to have at least above-average rigs to play on, it stands to reason CAS can be expanded to allow us to really get into detailing our sims. Again, I don't know the limits of the game itself or how that's determined, but it would be very nice to layer clothing beyond underwear and socks. To be able to tweak and tuck and pull and nudge the face, cheeks, jaw, chin, forehead, etc... in a much more detailed way than we currently have. I don't really mind the 'cartoony' look of Sims, but a bit more realism wouldn't hurt either, IMHO.
I feel like I'm asking a lot for a game that costs $40 for it's base pack. But when you start adding up all the expansion, game, and stuff packs... we're talking hundreds of dollars. For that kind of coin, it'd be nice to have something more in depth than what TS4 currently is, have more personality options, story progression, less bugs per dollar, a paint tool to help us match item colors across packs, and the ability to customize our sims more than we currently do.