Well, i thinked out and i think that a good idea is adding a difficulty select, "Easy" or "Casual" (easier than what we have now), "Normal" (the one that we have), "Hard" and "Extreme".
In easy mode, there would be more concentration of resources, and you would die with 3-4 plants attacks instead of 2.
In normal mode, it would be like what are we playing now.
In hard mode there would be a new item needed for entering to deep caves, a pressurizer, if you don't have this item and you're in deep caves you would start dying because of the pression.
In extreme mode... i didn't think out so... xD... but some ideas would be that the plants kills you in 1 attack, more fall damage, less oxygen and energy of the player... or something extreme.
I don't say a creative mode because that will be added in other updates... but i think that the difficulty selection and the pressurizer are good ideas.