It's better to work on a known engine than making an new one, you can do a lot with RV3 or 4, you would be surprise. To me it's the right direction to start with a proper base like they are doing now.
From what I saw in the warZ videos, it doesn't seems to be that good... I wouldn't be too confident about that game. Hopefully we will get something to play on before the end of the year.