I'd suggest starting with MCCC and a perusal of littlemssims tumblr page. littlemssims has a few million small mods that each tweak something different and what you will want is going to differ from anyone else. But their page is a really good place to go for an introduction to ways to alter the game.
Beyond this, if you want to make traits have more impact you start needing more intense mods. Things like Wonderful Whims that puts a whole romance system into the game. After that mention, most of the others have piles of NSFW changes that prevent them from being named here. The good ones have config settings that you turn off everything but what you want, or are modularly built so you can just install the desired pieces. But some of them are 'all or nothing' packages that mean you need to really think about it before installing them.
Otherwise, there are a ton of 'new traits' mods out there but they just add more traits and not alter how those traits work. Most of these turn your already 'super happy' sims into superhumans capable of everything short of leaping buildings and catching bullets... so, downloader beware - you really need to 'test' trait mods and delete a lot of them... and that will leave half your random townies lacking traits, so you will then need to either edit those townies, delete them, or start a new game.
And... the more popular the traits mod pack is, the more superhuman it is likely to make your sims...
Also gotta be cautious with 'CC' mods that add more things for build mode. It's not uncommon for these to have buffs set to '10' stacked on them. Things like a bed that has all possible bed buffs set at 10, and costs 0... or computers that are fun 10. These things can become like drugs to your sims - they'd rather die that stop using that computer.
And fashion CC, often has the 'flags' for color, outfit category, gender, and more set improperly or not at all.
Build mode CC also usually lacks proper flagging for style and color.
This will mess up your ability to enjoy things like the new 'likes/dislikes' system of Dream Home Designer. When you get that client that loves purple french decor so you build them a house that would make Prince proud, and they get a massive bad moodlet which you hover over and it says they are unhappy for being surrounded by grey modern furniture... which you don't see anywhere... until you open all that furniture in Sims 4 Studio and discover the maker never set the flags off default...
Or when your two 'loves pink country fashion' sims that are dressed in pink cowboy outfits break up, and it turns out it was because they were deeply unattracted to people wearing black goth clothes... because all that CC that looked like cowboys and cowgirls in pink, was labeled as 'goth/rock' and flagged as 'black' in Sims 4 Studio by it's maker for who knows what reason... (because until you're running a romance mod, your sims couldn't even tell what the other sims were wearing anyway, so those flags only mattered for CAS filters, but now they're super important to your gameplay...
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