I started playing TS2 for the first time recently, and to my surprise I've been really liking how it does lot travel. When you come back its the same time as when you left, and that way I can have multiple characters go out to different places in the same time span (unlike TS4) without having to juggle them both at once (unlike TS3). Highly doubt that's how they'll do it in TS5 though, sadly.
Since its not happening, I would be fine with TS4's system again or actually open neighborhoods in TS5 like other people have suggested. Actually open neighborhoods probably won't be too much to implement since as is all the other lots are clearly being rendered, letting us walk around in them probably shouldn't be too much more?