Sims 5 will likely be starting development around the last 2 or 3 years of packs for 4.
This is my speculation for what it'll be like.
Gameplay wise I would expect a blend of ts4 and ts3, sectioned off parts of the map like ts4 but setting for playing with open "hoods". Potentially very similar to ts4 (cause it was least lag enducing) with more lots in general.
"Major Game changes" I would expect are potentially
- online co op, a revisit to their previous attempt to make the game multiplayer
- an art style change
- perhaps a step into procedurally generated world?
