I'm a big fan of fostering creativity though restrictions, so this makes sense to me. I would be interested in any notes on how you try to realize that goal in the engine. It would be interesting to see the difference between limitations imposed by performance v.s. limitations imposed in the name of a focused creation experience.
In general, any time you're not "simply" (not that the problems are simple) creating a system that exists elsewhere (physics, liquid, etc), it would be interesting to hear your thoughts on where you choose to deviate from the 'norm' and why.
For other things, if I introduce some system I'd like it to be unique and have purpose. Many games do GPU-based particle physics for water, and the ability to have this produce any significant interactions isn't that great. My system is CPU-driven, and contains things that are rarely done in games like simulating pressure and a constant fluid mass, which makes for more interesting interactions (you could pour water down the end of one pipe to raise it at the other end, as just one example). Usually people find ways to modify the system that I would not think of (someone made a roller coaster in Minecraft using water canals IIRC).