I don't see "Global colonization" as necessarily being exclusive from the other categories, but rather more inclusive of everything else.
I cannot really share that point of view.
If your main goal is to create a certain atmosphere or even try to tell a specific story you most likely have to concentrate on a specific period of time or region,
add the aspects that fit and eliminate everything that does not fit into that.
For example
Werewolves from
Writing Bull.
The mod offers only one playable Nation but achieves a certain atmosphere and tells a story.
In
TAC and
RaR we even had lots of discussions about adding new Colonial Nations like New Denmark, New Sweden, New Scottland ...
Even though these Nations all had colonies they were still so unimportant for the setting of the New World that some people said they would damage atmosphere.
Another example is Civ4.
Some people think it is great that France can build the Pyramids.
Some other people simply feel that such things are absolutely unatmopspheric.
Too much diversity makes it much more difficult to create a specific atmosphere because it gets much more difficult to focus.
But of course it is not necessarily said, that authenticity or atmosphere are important for a mod.

(Personally I like to see these things in a game or mod though.)
I want a mod that is diverse, rather than narrow.
Well, there is nothing to say against your own preferences.
But there are certainly players that rather have less diversity but more authenticity, atmosphere or story.
For those players a "Global Colonization" setting might very well be exclusive to a "New World Colonization" setting or a "Medieval" setting.
The differences between a SciFi setting and a Medieval setting (from a purely coding point of view) would be the technologies and the economy, but these are mostly symbolic.
Not necessarily.
You could have a map script that generates many "small planets" instead of one World map.
Also in a SciFi setting you could have Nations (or Aliens) that absolutely do not fit into a mod that has a more Earth focussed approach.
There is very little that is unique about empire building sims.
I disagree.

I do believe that
Total: Wars,
Civ4Coloniatzion or
Galactic Civilizations all offer quite a different gameplay and several unique features.
The fundamental differences are in the economy of Civ4Col -- i.e., raw resources are processed into finished goods --
Agree.

The most fundamental conceptual difference of Civ4Col is the economical system (including the transportation system).
But even that could be changed ...
... but post "revolution" there is nothing but disappointment. The game simply dead-ends.
Agree again.
But it is the same with many other strategy games.
Once you have gone past a certain point in gameplay, there is nothing more to do than grow even bigger.
All the mods can be said to have been created from our disappointment with the vanilla game. We all see the potential to do something more with the game,...
Agree.

But each modder might see the potential somewhere else depending on his own interests.
... but we all have to admit to feeling a degree of disappointment that we can't populate out maps with units that look the way we'd like them to look.
I never had that feeling.

Mostly due to the fact that the teams I was part of (TAC and RaR) always had graphical geniusses on board or as partners that could create all the graphics needed.
(
melcher kürzer,
Fankman,
Willi Tell,
Schmiddie, ... all have great graphical skills)
It is simply a matter of skills you have or might need.
Some mods have problems getting great graphical modders, others have problems getting great programmers.
The American setting (in the broadest sense) has been fleshed out very well and I do not mean to detract in any way from that. But I see that only as the first part of the work, not the completion.
Let us formulate it differently.

It would be great to have more mods trying to create other scenarios as well.
In fact, I see many of the basic features of Civ4Col being so Euro- and Amero-centric as to verge on racism.
Please, let us not start with "racism" again.

(I am really sick of such useless discussions.)
TAC and RaR have done much to reduce the "white man wins every time" aspect of the game, but I want to take that further.
Well thanks.
TAC wanted to create more atmosphere. (Graphics, Texts, Events, ...)
RaR focussed on creating a more complex and challenging gameplay (Features) on top of TAC.
As I have said, I want Native civs to be playable and to have the chance to "win".
That aspect is more or less independent of the setting of the mod.
You could try that in a "New World" or a "Global" or a "Science Fiction" setting.
It is simply a matter of the gameplay you want.
Summary:
I think all new mods would be great.

May they be "Global Colonization", "Medieval", "Science Fiction" or whatever.
I do see the interesting aspects in all of these potential mods.
I just don't hope that modders will try to stuff all of that into one single mod.
Simply because I believe that such a mod would need to make too many compromises and thus loose too much atmosphere and probably also quality.
I like to eat Pizza and I like to eat Steak.
But I am not very fond of getting a strangely tasting "Steak-Pizza".