I think a trade based system could work well: undermining neighbouring competitors.

I found out that gophers/moles will pull carrots from below the soil, planting the seeds in the direction of a neighbouring fence the burrowers could be directed away from your garden, the tunnel then flooded with water, which would cause a headache for that neighbour (sort of like getting a "Tetris" which pressures your opponent).

All in the exaggerated spirit of multiplayer of course !