Personally I think adding the ESP8266 directly onto this board would be a good move. It can be designed to use either an on-board antenna or an external one so you don't lose anything by doing that other than having to purchase another chip. I've seen other companies doing this so I wouldn't be the first.
I haven't done a Kickstarter or Indiegogo campaign before but I've designed and built many production boards. There seems to be interest in this board but enough have to be built in order to keep the cost cost down so if anyone has any suggestions I'd like to hear.