Something that slows the rate or crop growth, maybe being able to assign 12 24 36 48 hours for a full from seed to harvest. Along with a surface only farming, to prevent level after level of underground farming. Just to add a touch of realism.
I think the ticks of growt aren't being handled very well in the server. At least I read a lot of problems with adjusting it. This could be accomplished by looking at the lightlevel of the block I think. Because the outdoors give more light than torches or glowstone can give.