I think I’d like to see more trait options and slots like the sims 3 and also have a memory system like the sims 3. I don’t remember how the memory system worked exactly in TS3 (had that game on a now broken disc several computers ago lol) but I remember enjoying that you could easily look back on your sims experiences that really made them feel like they had a past.
I think they might be headed in the right direction with sentiments. I like that an experience with others creates sentiments (that feels meaningful to me, but it would be cool to see trait based sentiments too since the base game ones really feel a lot like the identical emotions systems in that everyone is forming the same sentiments). Sentiments affect emotions/buffs too - like getting shared sadness when someone they share a certain sentiment with gets sad (love that, definitely the kind of complexity that I’d love to see more of). I’m not sure how long sentiments are meant to last or if they can get strengthened. I play rotationally and sometimes they are just gone the next time I play a family.
I would also love to see a buff system that takes experiences, sentiments or relationships into consideration, like a lasting resentment toward ex or a happy moodlet from spending time with a best friend/long time friend. I’m not sure if something like this would be possible, but it could be cool to see sims react to past experiences and places too - like let’s say a sim watched someone die while they were out on a community lot, it would be neat to see them get a negative moodlet for a certain amount of time afterwards since the location might trigger bad memories. Now I’m just rambling, but giving Sims lasting experiences and memories with consequences (both good and bad) would make them feel so much more real
