Hi!
I am with the faction who don't think that forcing you to trade is a really good idea. I especially do not see the plausibility of the defective culture you suggest by saying they need to be lacking important skills/resources.
I do agree that the current abundance of resources on site is a problem as it considerably takes away from the personality of the site. And given that you get metal from the humans and metal and stone from the dwarves, lacking one or more metals/stones would not pose an unbeatable obstacle, I think (especially if you take into consideration that you can increase your embark points, thus allowing you to embark with some extra goods if you need them in the first year).
Requiring more space for farming is a change I am less fond of. First of all, the real size of a tile is basically undefined - only one dwarf can stand in a tile, but dozens of dragons can lie in that same tile. Thus, the argument about relative size becomes kind of moot, especially if you look carefully at the sizes of different objects in the game (a well is one tile, as is a table and as is a chair - right along a coffin! Usually, I would expect a coffin to take up at least twice as much space as a chair, even if you give some space for shifting it).
Secondly, it can quickly make farming nearly impossible: Remember, you need a complete rectangular area of farmable tiles in order to get any crop, so you either need very much luck for getting that area, or you would have to define 200 tiny farms and set each of them for the individual crops.
Third, unless you have a very leisurely fortress, your dwarves are already quite busy. By increasing the number of farm tiles, you require more actions for planting and harvesting, which means more dwarven labor is required. So, you may end up with either a farm fortress where nearly all dwarves are just farmers all year long, or you have a trade fortress with no farming at all. Certainly a diversification if compared to a multi-talent fortress, but it hinders experimenting with mixed forms and limits player freedom.
Making plants take longer to grow, increasing the importance of fertilizing seem more reasonable options for me to make farming more difficult while not making an outright impossibility probable.
Also note that the non-seasonal nature of the surface plants makes farming very easy and also reduces the flavor of the game (there are no vegetables of the season, so to speak).
Deathworks