There's some poking in the inspector to get ideas, but there's more skimming over sprites... and yes, it's mostly whimsy. I have my old
to-do list, but the last several things I've added appeared nowhere on it. Honestly, my normal decision-making process involves rooting around in a sprite dump to find some specific image I can use for a particular purpose, spotting something completely unrelated that would be interesting/ridiculous to add, and dropping everything to go off on a tangent doing that. E.g., a few weeks ago I stumbled across a Wikipedia article on insular dwarfism among extinct pachyderms (don't ask me how or why; I no longer have the slightest idea); NG now has pygmy elephants and mammoths (as well as dwarf elephants) for pygmies, hoburgs, and vaettir.
On the minus side, lack of planning and running on interest rather than structured plans means I've got a bunch of in-progress additions that have never been finished. On the plus side, I've kept adding
other things because I'm usually at least reasonably enthusiastic about whatever garbage I'm cobbling together at any given moment, so I've managed to add a lot without burning out on the project.
[That's all in reference to my part in the project, which is mostly just content dev (though coding happens occassionally). The
Issues page of the Github repository is probably about as close to the overall project having a roadmap, and that's mostly just a pile of ideas or things one of us thinks should be addressed, maybe, eventually...]