I'm enjoying the server, thanks! One of the reasons I wanted to try it out is to find a more challenging experience. My suggestion would be to include fewer mods (or downgrade some) to keep it a constant challenge. Things like No VIP perks (recycle/drop), fewer set homes allowed, basic loot tables, no pipes. Even a max number of revives per 24 hours -- or make revive number 2+ so darn expensive to buy that an in-game death is something you seriously have to work hard to avoid.
I also like the other suggestions for upping zombie spawn rates and night hordes.