They could implement an ask before you engage kind of option. For example: A player wants to interact with another sim, a pop up would state that they want to chat or what not. Yes or no?? To stop multiple boxes popping up you could disable it or you could just allow sims to just talk to your sim whenever. The whole point of the online aspect is for your sims to interact with others, I think it would be pretty funny if another sim came up and started fighting lol. thats where your already gained skills come into play.
Sims 4 basically revolves around skill building anyway and the graphics as they are would probably interest a lot of people out there.. For example Fortnite's graphics are similar and thats a huge hit. They could introduce new gameplay such as mini games for your sims that play off their skills. Sports games and other little competitions to win prices to use back home.
As for CC just dont allow it in the online bit? They could bring more CAS options with the expansion and before you visit the world you can change your sim so that it wont conflict. But when you return back to your home the changes reverse?
It seems like a lot of people saying no to the idea don't like playing with others, and thats fine. But some players would love to, even if its only 2 player. The thing with making an online version is that it wouldn't interfere with your solo version at all apart from maybe to bring home some new relationships and gifts etc.. And you know, there's always the option to not buy it.
If sims 5 is even going to be a thing, do people really want to just go through numbers? Sims 4 has a fantastic CAS and build mode in my opinion, more players can actually play the game without having to buy a new PC. Each year the expansions have gotten better. Bringing online to the table could open so many doors and opportunities.
Online could even be completely separate. A spin off.