I want them to take as long as necessary to make Sims 5 (if they're going to do it) and do it right. No object babies, no waiting 2+ years for toddlers, children who have play options that don't require "stuff", teens that don't feel like mini-adults (the mood swings from Parenthood should be in a Sims 5 base game), more definition between YA and Adults, and Elders that aren't just waiting for Grimmy to show up. Open world if it's done right -- maybe just the world you're in is open then you have a loading screen to travel to another world (like Willow Creek would be open but you'd have a loading screen to go to Oasis Springs which would then be open when you arrive.) If they start with the premise that it's going to be a single player sandbox game rather than some sort of online hybrid, they can develop the game with that in mind rather than switching courses midstream.
In the meantime, there are still content ideas for Sims 4 out there that can be explored. Magic, University (or improved education at all levels), Rural Living (maybe combine with "off the grid" features), Celebrity (as long as every Sim doesn't turn into a celebrity like Sims 3), more vacation destinations (build/run our own resorts?) -- the list goes on.
