Considering that you can expand the Farmhouse, Coop, and Barn, a Greenhouse expansion would be great. And while we're at it, a basement for the Slime Hutch would be good to hide your slimes away so to protect their natural color from the Witch's unannounced goth makeover visits. She's really quite the insistent slimedresser.
Iron Bars, Wood, Clay and refined Quartz seem real good resources for Robin to work with and it'd probably have to cost similar to the final Farmhouse upgrade because it's such a game advantage. IMO adding farmable spaces in a Farming game where there are already so many farmable spaces doesn't seem to be unbalancing as those things are always a consideration to a game designer. I don't think the expansion should be to more than 8 (2x4) or 9 (3x3) total spaces covered by Iridium Sprinklers as more than doubling the space is teetering on game-breaking.
Because the final acquisition of the Return Sceptre needs truly to feel like the apotheosis of a long, sore, filthy, tiring, slimy, bleeding, stygian, penniless, crusty grind to the true Final Boss of this game. You can't cheapen that experience by making it too easy or you'll not appreciate the magnitude of what you've done.