I can get behind that. It could also be a lot more in-depth than Sixam. But the only way I can see the weird plants and such happening in a sense is with the new Realm of Magic pack, though it may not be what you described. It would have been great if we could live on Sixam. It was so intriguing to me when looking back at the EP, but also very limited.
I'd add some ancient powers into the mix.
*Cue eye rolling* Lol.
As in, mind control and whatnot for my Sims to use against aliens that either do them/the world harm or for my Sims' own selfish reasons. The catch is, they can only obtain said powers whilst in that world, otherwise they can just blend in among the locals. Each individual power (1 per sim) is learned about from a specific artifact held by Elder Aliens (i.e. Similar to those crystals you can collect in several worlds) that glows whenever your Sim is near it, and is earned by whether or not they fail at a certain task given. If they do fail that specific Sim is not allowed to re-take it. Ever. Because consequences are far too underused. This would all be optional of course and non-liner, more for people who like that sort of thing as it might seem too silly to everyone else.