I have two things that are a must (or I won't buy it at all) and two things that I'd love to see in the base game (iffy on not buying it if they're not in it)
the musts:
A color wheel or color pallet - EA can make their own colors as well, but at very least I'm going to need an option with about 16 basic colors that I can recolor most of the items in whatever I would like to color them in. I can do without create a pattern, I liked it but I understand that was part of the drain on Sims 3.
Semi-open world/Open Neighborhood - Realizing again that a fully open world was part of the drain in Sims 3, I can understand if we don't ever see fully open worlds again. I would completely happy with open neighborhoods with up to 6 lots each.
Without both of these, I see no real reason to switch from what we have in Sims 4. Ever.
the big wants:
Basic weather in the base game - almost every other "big budget" simulation game has basic weather in it. By that I mean, rain and snow. Just give us rain and snow, EA. Not every day has to be a sunny day of happiness and I'm tired of paying for, and more importantly, waiting for several years for, any kind of weather options. If it raises the price of the game, so be it. I'm much rather pay a bit more for it upfront than wait for it for any amount of time. More involved seasons/festivals/holidays etc can come in a pack through, that's fine with me.
Basic pets - I know this is a big money maker, but at this point, I think the idea of a cat/dog EP should be retired and should be in the base game. Again, it's more an issue of waiting for it for however long than spending money on it. As above, if the game needs to cost a little more to add cats and dogs from the start, so be it. An extended Pets EP could add in more types of pets.
These two themes represent what I feel are the most integral parts of life. It's not exactly activities, pets and weather can have a big impact on our real lives, and having them in from the start would, to me, make the game feel more alive on day one.
If we got the two "musts" and only got one of the two of basic weather/basic pets, I would be interested in buying it eventually, probably on sale, because I'm sure the other would come eventually. Getting all four things from base launch though, I think there's a good chance I would get at least the base game on day one.