Updating the UI should be a main priority.
With every expansion added, it just keeps getting worse. What I want them to do is look at the full game with all expansions installed, and see how they can make it more manageable.
I don't have all expansions, and the UI clogging up further is also plays quite a part in what makes me hesitant. Because every expansion does add a new element there.
Looking at the Simology panel, I can do without seeing my sim's preferences, romantic preferences and milestones there. I have to scroll past all that to get to their fame and reputation level which I do prefer to check on sims who try to get famous. (by default all my sims stay out of the spotlight.)
I think it would be better to move those preferences and milestones elsewhere on a sim menu, like you click on a button to get all that, leaving the simology panel only for things you regularly check on. This could be put in a new sim panel where we can write their biology too. You could find all the info on that sim there that you won't be checking on all the time. This whole panel should only pop up with a button though, not be part of the regular UI.
Luck is something that I don't think needs to be in the simology panel at all, we see a clover icon when their luck changed, why not have that icon just show their state of luck? It could maybe be tucked in somewhere small, it just needs to be an icon we can hover over so you get the info about it, but if you already know the icon alone of a four leaf clover, regular clover or rotten clover should be enough.
On the skills panel, I think a lot of space can be saved, several skills could show in a row next to each other if you'd just see the skill icon, the progress bar next to it which could be shortened to the length of the bar above a sim's head, and the number displaying the level of skill could be shown in that bar. What the skill is would already be shown when you mouse over it, the information it gives then can remain unchanged.
Skills could also be grouped together under the type of skill: Creative, mental, etc. Since the game does seem to group them in that way, and this can make it more clear what skills a sim should avoid or be doing when they are close to a burnout.
The pie menu is also something that requires a big overhaul. It worked great for earlier sims games which had less options, but it gets way too much here, and just gets worse with every expansion. It is a pain to find what you need and even if you know what you want and where it is, you might have to go through 4-5 pie menu's to get it.
