The mods I'd like to see are all somewhat leaning towards realism as that's what I think the game is lacking today.
1. A weather mod
I really think the weather should have a huge impact on both how and when you do work on your farm as well as for crop yeild.
This would make the game more challenging and not as predictable, some years would be really good, most average and some really bad. It would also change when you can do certain tasks, some tasks would be easier with rain others harder or sometimes impossible (harvesting).
2. Prices varying from year to year.
Demand for certain crops should vary to some degree from year to year. It would be really nice to have a message popping up with a prediction for next years prices so you could plan what you'll grow that year accordingly. For example "The Farmers Association predict a huge demand for wheat next year. Canola and oats are predicted to remain stable and cornprices are likely to drop next year". Not all crops would have to be mentioned or even affected by the price changes. This would also give the game an element of strategy and planning that is lacking today.
4. Specific seeds for specific crops
Seeds should be buyable and filled in your seeder/planter by specific crop and not the generic thing we have today.
5. Repairs
Repairs should definitely not be simply linked to the age of a vehicle. The number of miles/km's to it and most of all what you do with it, is what should matter. Since stones can impact the repair value then why can't ramming your tractor into a wall or colliding with a car do the same but with greater damage?
Even better would be specific damage to your vehicles that could be repaired in various ways. Either in your workshop during off season, or for more crucial damage in the dealership workshop, for stuff that would render your vehicle inoperable.
6. Moisture and rotting crops
Your crops should be able to rot or go bad in other ways if left unattended, or treated in the wrong way.
There should also be a delay after rain when you're harvesting. The yeild penalty should be gradually removed depending on the weather following rain. If the weather is really warm directly after rain, then the delay would be shorter.
7. Hands and legs in cabview
Get rid of the ghost driving my tractor and animate hands steering and legs pressing the pedals.
8. Small scale/manual implements
It would be really nice to have to handheld implements for really small fields. On some maps there are nice, small fields around the farmhouse but really no way of growing and harvesting things there in a way that would feel realistic.
9. Bankruptcy
A mod to simulate repossession of equipment would be nice. A don't really know how that works in real life but I don't think anything happens at all if you go bankrupt in the game right now. Perhaps the bank could give you a choice on equipment they need to take or offer you to lease the stuff while also repaying the bank.
10. Game speed control
For example to be able to specify that I'd like to run the game on x25000 through november-december and then pause the game.
11. More equipment from Deutz-Fahr
12. Mulching
I read somewhere on the forum that real farmers don't really use the mulcher every season. So why not have this working the way lime does?
Perhaps this is something that'll change with precision farming.