As everyone has said, the nani12 is a really great stone. So if you're not sure what to do next, you should ask yourself whether you're looking for a good shave, or you love razor stuff and just need an excuse to buy more.
If the former, spend your time getting the most out of what you already have. If the latter, I'm afraid you will need a lot more than just one finishing stone. You'll need a Thuringian, at least 4 cotis, some J-nats, a gok20, shaptons, anglo-stones, US stones, a zulu...