I read an excellent thread on the roguelike development newsgroup titled exploitation and evasion. As one poster says "we can model players as attempting to maximize the probability of winning even when it does not maximize fun." therefore I have a duty to make optimal play fun.
Optimal players are only willing to go to the next level if the reward per risk, on that level is greater than the level they are currently on; even if the rewards on this level are tiny, the greater risk of the next level may not be worth it.
Naturally I want each level of the game to be more difficult than the last and at the same time I want the player to explore the whole game rather then staying on the first level forever.
Since my game is centered on farming, the time spent on a level can be spent growing the resources to make the next level less risky. To prevent a player from camping on that level and farming forever, plants will exhaust the soil, making it necessary to venture further and collect nutrient sources or farm in a new area. A cycle of seasons would further encourage exploration by making times when you cannot gain anything by staying still.
But still, I do not enjoy games where I already know the one thing to do, and the only obstacle is frustration with doing the same thing over and over. Growing things should require at least a little improvisation. The simple way to do this is with pests. Say you are growing some bomb plants and some swarms of beetles keep eating off all the flowers. You can fight off a couple of these beetles at a time, but if you want to grow more than one bomb plant you have to try something different. There are poisonous plants deeper in the dungeon that you could plant around the bomb plants to repel these beetles.
So as production increases in both scale and time there will be decreasing marginal gains. The one remaining temptation for grinding is to collect infinite resources. To prevent this I will have only 26 slots in their inventory, the remaining items will have to be left in stashes. Items not in someone's inventory will be slowly decomposed by microscopic creatures therefore a player cannot spend all their time gaining resources and have to progress in the game at some point.
