There will be never a new Whatsapp app for N9, there is not even one for Sailfish sadly, nor a good one for Telegram (the original developer of Sailorgram left). The only trick you could use is to have an always connected Android phone with Whatsapp running on it and using Whatsapp Web on N9. But, even if I was also using an N9 and liked it a lot, it makes not much sense.