As far as I know there will be grass, corn, wheat, rape and barley for now.

And when you watch the design of the 'dynamic' fields, i dont think we can create our own fields in game like in fs with a plow. They're a kind of a plane that you have to add in the editor. In game you see the boundaries of that plane very well.

If you look at this trailer youtube.com/watch?v=Bq9upQNVxn0&t=144s you can see a tractor plowing grass so I am guessing you will be able to create your own fields? Also under gameplay options there is something called field borders, but we are not able to disable that yet so who knows what it does.