Try and make New Zealand a welsh-speaking country, with at least 50-60%, or more, of the population speaking Welsh. If it involves having just the South Island or just the North Island speaking Welsh that could also be interesting.
Oddly enough, very few Welsh immigrated to NZ during colonial times, so far as departure records are concerned anyway. During colonial period settlement was targeted at Ireland, Scotland and England, with the latter suppliyng the lion's share but not as much as you'd expect given England's size.