Does it need to be a nightie? I find 'traditional' pjs like this style (ok, maybe not that pattern! but wanted to find you an M&S link), or ones with buttons down the front the best.
Also, if you will be breastfeeding through the winter cold, think about long sleeves. Especially if you might be co sleeping and having to push the lovely, warm, oh god how I miss it duvet down to your waist.