I honestly didn't care for the open world design in Sims 3 at all. It came at the cost of playable venues (rabbit hole buildings) and completely lacked a decent rotational play mechanic (which was wedged in later and turned out just awful and unmanageable.) It bunked my entire play style. The worlds were also too big to be manageable so I got annoyed trying to play in them, and it was extremely time consuming for a Sim to go anywhere. Plus, there were hardly any Sims showing up at public venues so these great, giant worlds turned out to be ghost towns and really fell flat. And finally, I really couldn't stand being imprisoned in one, single world environment and unable to move my Sims to a different one without giving up everything and everyone they knew. It felt like a downgrade from Sims 2.
Sims 4 has its problems but I'm actually pretty happy with the world designs. I love the backgrounds, the variations of the towns, the open neighborhoods, the ease of being able to move them around to live wherever I want, and the simplicity of managing my households and populations. I don't think I'd want fully open households in each neighborhood because it's not necessary. Since I fill the neighborhoods with my own Sims, I don't want a heavy story progression that's going to alter and marry them off. I already get really annoyed when the game randomly selects my Sims for filler jobs like bartending or food stands.
But I think a future game could be designed to have slightly larger neighborhoods with more lots and a story progression that only affected non-played households. Something that works like the design of Jungle Adventures with the gates could make it feel more open and eliminate some of the load screens by only 'activating' lots when your Sims are near them.
