As promised, here is the over the top stuff. This could probably be fun, though, if it's simplified to the point where you don't have to have a farmer's almanac next to your computer.
First of all, make grapes tree-like rather than wheat-like. I've suggested some ideas on how trees could be handled in my Trees expanded
thread. Grapes should probably grow faster than any tree, though.
Multi-stat crops, inspired by cattle. Crops have three stats: quality, yield and hardiness. When you plant a crop you choose which of the three you want to raise, and any improvement points you get go to that stat. The sum of all three is used to determine total crop quality for purposes of further improvement. So, you could just raise quality and get really good flour, but you wouldn't get much of it and your wheat would be really sensitive to sub-optimal growing conditions. Or, you can balance all three and get crops that are decent at everything, but don't really have an edge in anything (since total of stats is used for softcapping, a farmer focusing on quality will make flour that beats yours hands down). Now, if we really want to go crazy, we can break these up into substats.
Quality does pretty much what it does now. It could be broken down into qualities for individual plant products - straw and grain for wheat; fibre, seeds and flowers for hemp.
Yield is exactly what it says on the tin. The higher the crop's yield stat is, the more products you get per tile of the crop. Yield could be broken down into yields for individual plant products, just like quality.
Hardiness is the measure of how resistant the crop is to poor growing environment. As Loftar said, the plan is to model soil nutrients, PH and stuff like that, and I believe that seasons are also planned. So, all crops will have their comfort zone, ie range of conditions in which they thrive. Hardiness extends this range, which means that you could grow a certain crop where it wouldn't grow naturally, or you could squeeze in an additional harvest into a year. Hardiness could be broken down into temperature extremes resistance, nutrient imbalance resistance, PH imbalance resistance etc. If we really want to take this into sim territory, have separate resistance stats for cold and heat, nutrient deficiency and over-saturation, and acidity and causticity.
Now, all these stats allow for one fun thing: hybridization. If you plant two different strains of the same crop close to each other, they will produce hybrid seeds when they ripen. Hybrids have stats that are the average of parents' stats. Going back to the stats raising part, maybe it would be a good idea to make certain environments more conductive to raising of certain stats. That way you'd have to buy seeds from far-away places to make that perfect hybrid.
Expanding on the hybrid thing, hybrid vigour could be introduced. Vigour could result always, when the parent crops are sufficiently different, or when the parents fit into some arcane formulae (and if they don't, you get the opposite effect), depending how hardcore we want this to be. Basically, a hybrid gets a bonus to it's stats. To counterbalance it, only apply this bonus to secondary products and give seeds grown on a hybrid plant a penalty, which could represent the hybrid's genetics falling apart in the second generation. This would mean that you'd have to get new hybrid seeds over and over again, which would make it possible to make a living selling hybrid seeds to farmers.