Expanding traits is really something I'd like to see done, and I honestly feel like the whole like/dislike system needs to be reworked and should include food. I like that in Sims 3 they can have a favourite meal. Would be neat to also have sims dislike certain meals. I'd love to see it interact with traits too! Like nice sims will politely eat it anyway, but a mean or evil sim could make a scene to show their disgust and would like whoever who made it less.
It honestly could play nicely into the grilled cheese aspiration too. A sim could have grilled cheese as their favourite, and hate all other meals.
But generally, I do think they really need to work a lot on improving traits. Like maybe tackle a few each update, or maybe just one in great detail. But either way I'd want them to really give them all a lot of attention so that it is not only on par with Sims 3, but an improvement. Having two traits less than Sims 3 sims really hurts, I feel like it should do more to compensate.
Let's talk about geek sims for example. They should be able to really interact with Journey to Batuu in different ways. I'd want to see them geek out when walking around there, have interactions no other sim gets. Also I want to see them get unique interactions with video games and other nerdy things, and get more specific wants when it comes to nerdy hobbies. Like they don't just want to play a video game, they want to play a specific one. More games would be nice too by the way, really throw a lot of EA's game library in there.
I also think it could be nice if something like the wishing well might add a wish depending on traits, might not be exclusive to one trait by the way, and traits could also disable some wishes. An evil sim is not going to wish for world peace. They'd wish for world misery. A glutton sim might just wish for a really nice sandwich. A foodie sim could wish for a very specific delicacy. A lazy sim could wish for the perfect recliner chair. And so on.
So much fun things could be done with lazy sims by the way. I'd love to see them struggle to get out of bed as sims 1 sims with zero points in activity, lounge lazily on the couch, and maybe do various activities in a lazy way. They should also do that lazy twirl from Sims 2 when changing clothes, I love that little detail. Let their laziness be a whole lifestyle.
With evil and good sims, a lot more could be done too. Play it up a bit more. Let good sims be more of a mary sue, and evil sims be more evil. Let them be able to practice their evil laugh in front of the mirror, modify a rocket to try and blow up a satellite or do other bond villain stuff. Could be great for devious aspirations too. Some stuff that could get them in jail for a while, returning after a few days. And good sims could maybe try and convince mean or evil sims to change their ways, being naively hopeful about it. But it could happen if befriending them, their mean or evil trait could then become some random other trait. Could also be neat if an evil sim could try and make a good sim evil, could be an interesting aspiration too.
A genius sim could be able to make inventions no other sim can, creative sims could make crafts no other sim can. Traits could really play into what sims can create, adding options. Like the evil sim could do something evil with anything, even with making a bouquet of flowers they could make them stink or something. Mean or mischievous sims could also do that.