I've got an idea! Limited resources for (nearly) unlimited power generation!
Imagine an oil block, which appears near bedrock, it's flow spreads up instead of down, similar to glowstone, but the source block itself doesn't move, nor can it be moved by any means at all. (clicking with a bucket for example will give you a bucket of oil but doesn't scoop up the source, flowing oil can be scooped up similarily). You can keep pumping it for oil but you only get a reasonable amount. It flows up to the first free air space + a bit extra depending on how strong the source is, spreads sideways and from it there it behaves like water again, this would require three block ID's, but damn would it be nice. It'd however require a much more complicated refining process to be of any use. And a lot of energy to begin with.
Same for steam geysers. For something a bit more accessible. Just gotta place turbines on top.