thriftedambition Spiral staircases were mentioned to the team directly from feedback on the old forum (AHQ), EA let us know on a thread there. So fingers crossed we'll see them in the game at some point.
For extra landscaping items like plants and rocks, etc have you used the debug cheat? bb.showhiddenobjects and bb.showliveeditobjects (they don't require testingcheats to be entered first so they're safe for console players who don't want to lose console achievements). I just made an underwater rockery in Sulani on an ocean lot using the coral and underwater rocks and plants. I use the debug landscaping items in most builds now.