Built-in failure mechanisms...slow down the constructions, buy you some time to make the new art :))
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Destructive resources. Free lands can become a more real thing. Give the "poisonous herbs" a use. "weed killer" "fruit bush killer" "explosives" all seem like good things to incorporate in the cauldron, with the potential to mitigate or nullify its growth/maintenance. Undoing work, maybe not, but preventing/delaying it? Isn't that what MD is all about?
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ingredients where enough is enough and too much is too much, e.g.
250 sand, 250 water, 250 yellow candy = 1 layer mixed cement
but make space for 300 water, with appropriate implications for the stability of the structure (or just have it not work)
Maintaining balance between ingredients with time. This can be forced by making the growth occur in "stages" like layers of paint, you botch it you have to do it again, do another layer, or even other worse consequences. Maybe the balance is humanly maintained, and if you botch it it peels off three layers later.
The current lack of immediate feedback (need to manual refresh) would make this much more exciting.