I'd be curious about the kinds of themes that Pokemon tries to go for - specifically the addictive, 'catch 'em all' thing - applied in a new way. Collecting and trading is a key mechanic of the series, and there's surely unique ways to go about that. Maybe instead of a rote checklist of things to catch, what about a game where they have a mix of pokemon - old, new, and freshly-created - and just don't tell you what they are, where to find them, or what they can do. It could be like you said, a primitive person in the pokeworld, or a kid who doesn't have a "professor oak" to get them on track. Either way, it would embrace more of a "thrill of discovery" kind of thing more than a simple, rote "fill out the list". Imagine just travelling around in a big, Skyrim-esque overworld brimming with hidden caves, deep lakes, and secluded forests, and you have no idea whats around the next bend in this savage, uncivilized world, like a pokemon madagascar or galapagos. You could catalogue all the new and thrilling species, and have to do more to learn about them than just catch it once - maybe engage it in combat, or stalk it to watch its habits, so you can really earn that 100% pokedex.