I can help you out with that. I modded the spawns.xml for my server. I increased the ammount of wondering zombies found out in the open. Increased animal spawn rate in order to balance it for dedicated server, as the game does not do that at this moment. I also made it so that at night you have 5x as many zombies wondering around so it makes going out at night extremely dangerous.
The biggest feedback I got from my players and new players that joined the server was, wow this feels a lot more like A16, whatever you did this is way better than vanilla settings.
So if you need help editing that specific XML shoot me a private message and I'll be more than happy to walk you through how to properly edit that file.
EDIT: or if you want I can write up a "modlet" for you. this way you do not have to mod yourself.
I can make "modlets" for z spawn rates like low, med, high, max etc.