Interesting question @lin_d_hop, we have worked a lot and are currently still working on it on the interoperability project Data Food Consortium. Because the interoperability issue is not only between OFN instances but also between OFN and other platforms, as lots of producers do sell to hubs using different systems (OFN, The Food Assembly, and others).
So the learnings from the consortium are:
1- There is no way one taxonomy will fit every need, and lots of food hubs want to keep with their existing taxonomy, or be able to choose their own (especially when we start talking about standalone website plugged on OFN instances. I have a concrete example with the french potential big user that we are still discussing with :-)).
2- We already had various requests also to be able to have sub-categories, which today is not possible in the OFN as all categories appear on the same level in the list.
3- In order to make various platforms interoperable we need to find some form of “pivot table” so that we can “match” in which category a product from platform A will be sorted in platform B. We discussed a lot about what should be that pivot table, and we came to the conclusion that it shouldnt be a table but a “facets approach”, meaning that to describe a product there are various facets and each combination of facets make a unique product identifier (of course you can then have a flat list of all the combination, but you can choose what are the entry point…). So every platform/hub can decide how they sort the products (in which of their own category) given the facets they want. Some might have a dedicated category for organic products, some might have two categories for vegetables (root vegetables and leaf vegetable), or some just one “fruits and vegetable” category.
I don’t think we should try to have a single taxonomy, for me we should aim at making it possible for every hub to define their own category, and just build that pivot format to interoperate them.
Here are some reflexions on that:
- the three options for pivot table that we investigated before reaching a consensus on the facets approach : https://docs.google.com/document/d/1TBB94PNZCndIY7ezuWWbpAqET0eF5sf0WxhvJNJfauw/edit?usp=sharing
- you can look at the “facets approach” tabs to see in short what we have in mind (will become more clear after the first prototype is built) https://docs.google.com/spreadsheets/d/1kARqg1wXEYXOzNNaft8V2_0Ox0s2kNnyC8U9MZu4r1E/edit?usp=sharing
- and look at the blogpost I wrote where we shared the business concepts and ontology work (I have not detailed yet the product ontology, but it is described in the business ontology itself, even though we have not precised the properties / values) http://datafoodconsortium.org/blog/building-a-common-language-our-semantic-business-concepts-model
To answer on the French ontology, we had to go more into details and fully customized the taxonomy for our own need and what was more logical / asked by the local users. We have an ongoing discussion on the French forum so that if there is an issue with a user he can ask a modification, but any modification in the taxonomy is really a mess as it impacts all the existing products. So that’s why I think every hub should have its own taxonomy and we should just a well though pivot taxonomy that enable the information to flow in any direction 
I’ll share our taxonomy on the file later today, but wanted to share about the interoperability reflexion as you open the topic Lynne, if you are interested there is a dedicated channel on Slack