I have a vague worldbuilding idea, inspired by Tim Hutton's Linear Enzymes simulation (and some other related projects of his). Atoms can exist in various states, but can only transition from one state to another by the action of enzymes, groups of atoms bonded together in a certain shape that encode the initial and final atom states, bonds, etc.. The action of enzymes is just assumed to be a law of physics. Such a system should be complex enough to sustain life, assuming the enzymes are reasonably flexible. Now departing from Tim Hutton's version, let's require that all reactions, and the laws of physics in general, are reversible, so the laws of thermodynamics apply, then any living thing needs somewhere to dump entropy, making the design of organisms more complex and more similar to real life (since now they need to eat for more than just material to grow with). This system doesn't actually need an external source of power like the sun to sustain life though, so long as we allow the atoms to take infinitely many possible states. In order for enzymes to be able to specify the states uniquely, more complicated states require larger enzymes (perhaps the states are labelled by strings of letters, which the enzyme includes in spelled-out form). There is then no upper bound on the entropy that a bounded amount of matter can contain, but the higher the entropy, the more difficult it is to add more.
Suppose the world starts entirely full of atoms of a single state, state "0", plus one organism that eats state "0" and produces a mixture of states "0" and "1". This has higher entropy, so the organism is able to use this change to power itself and replicate, spreading out into the world, leaving behind it a patch of waste containing "0"s and "1"s that it can't eat. At some point, a mutation arises, producing an organism that can eat "0"+"1", turning it into "0"+"1"+"00"+"01"+"10"+"11". It goes back over the "0"+"1" left behind, turning it into its own even higher entropy waste. This cycle can repeat indefinitely, and the higher-entropy the matter becomes, the larger and more complex the organisms requires to use it. Perhaps a mutant arises at some point that is able to adjust its diet dynamically based on the local conditions, so it can life indefinitely with no more mutations. Life continually slows down as more negentropy becomes progressively harder to get, asymptotically tending towards 0 rate of activity, but it never actually stops, and there's always infinitely more to go. Of course, there could be predators too, but any ecosystem would have to include at least some primary producers, capable of pumping ever more entropy into the surrounding matter.
I have not worked out the rules for the system in detail. I don't think it really matters. I do think that designing an organism to live indefinitely in such an environment would be an interesting challenge, but also much too much effort for me to actually bother to do, especially since actually simulating a large enough world for such an organism to live in would likely be prohibitively computationally expensive so I couldn't actually see it in action. Still, I think the basic idea, of a world which is bound by thermodynamics, but bound a little less tightly than our own so it never actually has to die, only slow down, is an interesting one.
