As several mentioned, the features of a greenhouse can already be achieved in sims today, although the look of a greenhouse isn't perfect. Better build items for a greenhouse I would love however. I don't even think this would need to a SP, but a kit would actually work quite nicely. What you'd need is some glass walls, more variations of plant boxes and some greenhouse lighting and possible some wicker furniture - that's not that many items. Maybe two-three mini greenhouses as well for those without the space to build a big one, and a few new plants.
I also love the idea of orchids as collectables!