I would love for the Sims 4 Plant Sims to be their own occult. Sims 3 had them down pretty well. Although I would love to have a choice between leafy hair or actual plant-colored hair that would complement their skin pigmentation; such as black, different green tones, rustic brown, brilliant orange, radiant red - don't forget the eyes. In Sims 3 they didn't get pregnant, but instead was placed a plant sim seed in their personal inventory which had to be planted and nurtured till the baby was ready to come. If a non-plant sim and any other type of sim were the parents, be it human, vampire, or whatever; that the baby plant sim would be connected biologically to both parents.
If they dried out they would not lose their life state, but rather their life, and that they would remain in this life state through the duration of their total lifetime. Their water need should not drop quickly, but rather slowly over the course of the day, meaning that if they took care of their hydration need in the morning; drinking water, taking a shower or bath, swimming, etc, they would be good until 24 to 26 hours later.
In Sims 2 there was a special light that a plant sim could use that imitated the sun, that would be nice for this to be brought back as well. Sleeping in the ground would be the same as sleeping in bed, the large planter boxes should work for those who live in places there is no yard to sleep, thus the avoidance of winter issues.
Plant Sims should have leafy outfits in all outfit choices.
Not to forget any plant sim interactions, there were quite a few in the past Sims games.