Edit to my original comment about functional houseplants, I have created a list of features/interactions:
-Green thumb trait: This trait would give them a considerable advantage when getting into houseplants and taking care of them. Higher success rates/knowledge, and more affected by what happens with their plants/buffs. (Reward Trait: Plant parent). Reward trait will give them high success with treatments/whatever they do with their plants, the very minimal chance that their plants will die.
-Skill: Could be called the "green thumb" skill and have it be a mini skill like ice skating. The more knowledge you have about houseplants (could be earned from reading a book or just taking care of them and learning as you go), the better you get at identifying their issues like when they need to be watered, what pests they have and how to treat it, what to fertilize with/when to, etc. Higher skill levels could take cuttings and even make hybrids/crossbreeds similar to the gardening skill.
-Growth: Having a couple of stages that the plants go through, (seedling/cutting, immature plant, in-between phase, another in-between phase, and fully mature plant). The ability to take cuttings from a plant once it is fully mature and then plant it in a pot to start the process all over again with that cutting. You could also have things that could speed up or slow down the growth of the plants. Things to speed it up: fertilizing it, a humidifier, a grow light. Things to slow it down: off-season (fall/winter), pests, non-filtered water, etc. Different types of plants grow different so you could have trailing plants like pothos or heartleaf philodendron that eventually trail once mature enough, or climbers like the monstera deliciosa that get bigger leaves and fenestrations (holes) when they mature.
-Watering: having your sim need to water them after a certain amount of time, maybe customized to similar watering times like IRL? Leaves turning yellow and then brown when not being watered/neglected. Could have the option to water with tap water or filtered water, filtered water could speed up the growing process.
-Pests: A need for a pest solution to be applied You could have two different options like "natural pest repellent" and "chemical/not natural pest repellent" and each would have a different success rate along with the natural one being free and the non-natural one having a price. Some possible pest ideas would be spider mites, fungus gnats, mealybugs, etc. Could also have lower green thumb skill leveled sims have an interaction like "treat for unknown pest" which could have a low success rate, and when they are higher in the green thumb skill could be able to identify what pest it is and treat it accordingly with a much higher success rate. Also, an option (although this may be much harder to do), have infestations if left untreated to surrounding plants if in a certain radius from the affected one.
-Fertilizing: You could have natural fertilizer that you could apply or a chemical one (much like the pest treatment, one could be free and the other have a price), and applying it would change the "quality" of the plant/growth rate.
-Pots: Cute pots!! (enough said, lol). Some nice terra cotta pots, or ceramic ones, hanging pots for ones that hang, etc. You could also have pot stands, baskets, or hangers that certain pots could go into/on.
-Types of plants: Monstera Deliciosa (climbing plant), Snake Plant (upright growing), Heartleaf Philodendron (trailing), Pothos (trailing), Cacti, and Succulents, etc. Each plant has a different look (some similar) when growing, like a snake plant would get bigger and then multiply within the pot, so say you start with 3 leaves, would grow another at another stage/get taller, etc. I have tons of other more high-level collector plants I'd love to see in-game as well!
-Buffs: "New growth!" - Sim becomes happy that there is new growth when the plant moves on to the next stage. "Pest infestation" - Sim becomes tense or disgusted. "Plant died" sim becomes sad that they were unable to keep this plant alive, maybe it just wasn't the right plant for them. "Plant propagated" Excited that they will be able to grow another plant from their older plants! Etc