I suppose it's probably obvious, but enemy variety needs to exist to shake up my strategy. As it stands, a stabber will trade with zombies at about a 1:1 ratio.
Basically, I would want reasons to use something other than a stabber.
Also, I guess more lanes would be interesting or longer/shorter lanes.
Different things that may force you to change strats up.
I would assume Plants vs Zombies has some interesting ideas in that regard. I wouldn't know. Never played it.