Realistically, though, how open to abuse would up to five NWs in a city be? Especially with forced decentralising of WW hoarding? Three is obviously great, but to get all five early enough to make a major difference, you're going to want a Hermitage in there, which will take up one of your slots. So, effectively, this is four NWs. Commerce boosting NWs want to run lots of Cottages, whilst NE wants lots of Specialists, so again the synergy is limited. An ultra commerce capital with an Ironworks is reducing its health cap by two population, and IW wants to be in a hammer heavy area to grab a bunch of the big late wonders or pour out troops. Stacking up military type NWs will mean sacrificing a lot of turns of unit production to get culture production online. Another option would be to have NWs start unlocking at Developing, where the border expansion outstrips the city's ability to work the tiles. I'd be up for playtesting the concept, at least.