I agree on pretty much everything
@WildWeazel has mentioned. I would, however, change the parameters from, "Essentials," "Enhancements," and, "Extensions" (although I do admit the three "E"s are pretty cool

)
I think we're better off with:
1. Using materials at hand to (a) the game's broken bits while (b) adding on anything which logically (both "conversationally" and "In a Boolean style.)
2. Whatever extended game play components are desired.
3a. "Extensions" which add significant new scope to the game (e.g., multiple map levels, as in Civ 2.)
3b. "Extensions" which require major changes to improve, like "Look-and-feel" items entailing Map rotation or significant changes to the UI.
I am not, in any way, opposed to these things, but I think that organizing them in the way Comrade Weazel (

) has isn't necessary. For example: "Gaining science or culture for exploring the map," would, I suspect, only take a simple set of algorithms to enable. On the other hand, "More contextual unit animations" would - I think - mean changing every .flc set in the game.
Also - for example - we were also tacking along parallel tracks about the nature of Colonies. (My approach was that, along with whatever City growth parameters - for example's sake, from Town to City to Metropolis - that a Worker could found a Colony, with its size limited to "1" unless/until it is joined by a Settler, at which point it becomes a normal, Size 2 Town. (Aside an aside, this is consonant with one idea I like - tat a Unit can only move
N number of tiles from a friendly City, until a certain Tech is achieved, then ... Well, I'm sure you can fill in the rest.
Perhaps a more helpful breakdown might be:
1. Everything we can fix that's broken, like making Artillery work.
2. Anything we can readily extend and implement, from that work, such as (off the top of my head) setting regular Patrol roues for units.
(For "3a" and "3b," see above

)
Of course,
What we want to execute and
How we go about doing it are inextricably entwined - But these are still very early days.