Just as a note - we did actually have Urban Zones in the design from the start, they're not a "half measure" reaction to desire for more generic building slots.
I mentioned them in the original dev diary.
I like the idea of general vs. specific in the same slots.
For example, the bottom districts could be combined into a single "Local Production" district set which produces Minerals & Energy. Add a building and it produces Food, too. But it's limited to 3 building slots, so when you want more efficient Minerals / Energy / Food then you want to swap it out for a specialized district type.
That would leave you with two extra District slots to specialize a colony.
For a Rural colony the specialization would be Mining / Generator / Agriculture -- just as we have now -- so there's nothing lost. You wouldn't lose access to those three district types. They just wouldn't be forced into every colony.