I don't really see the need for this. Teaching yourself how to mod in a new country is a 10 minute exercise at the worst. It's a different situation from CK2 where navigating DNA/property codes was nightmarish.
Teaching oneself how to mod is truly not difficult,but IIRC, most players are playing vanilla and may not want to learn modding, and I suppose this part of players have also their own thoughts about the game. It's true that generally players have always more than one choice when they want something in the game, they can simply post here and hope devs to adopt their suggestions or deal with their whining, they can ask a modder or join a modding group to make a mod, they can learn modding...But sometimes the thing they demand or let's say their need is not big enough to bother themselves to do any of above, and even it's such a small thing, if it can be satisfied, that's still very pleasing, nobody dislike a quickly fullfilled demand.
So why not another choice, a choice which can deal with their demands more quickly?
Not to mention the flag. CKII had a flag (or rather coat-of-arms) generator because it needed one anyway for the myriad barony-level holdings.
EUIV won't need one, and so probably won't have one.
Quid of your created country, then? Leave the flag out and be forced to play with a black flag? Assign it another country's flag? Let the game allow you to upload an image?
Nothing's simple and/or elegant enough.
A flag is a bit different from a CoA, but still, I don't think a flag generator is impossible: a stock of patterns and symbols, and a color picker and voilà.
And if there's a achievement system, the flag maker can also be associated with achievement: unlock a certain achievement, you unlock a certain pattern/symbol.
There are several problems. If you create a nation you also need the national ideas, decisions, possible names (culture) of the ruling dynasty ect.
The created nations use preset national ideas, maybe there can be several presets like "Land Power" and "Naval Power", or something like "Western monarchy", "Oriental monarchy", "Islamic", "Tribal".
They may not have special decisions but only generated ones, which ones they exactly have depends on selected culture group and so on.
Names depends on the selected culture, if you choose to create a Chinese rebel state you have Chinese generated names, and if you create a Mongol tribe you have Mongol generated names.
And maybe you can simply type a dynasty name for your new country, if it ends up with no heir, you get generated dynasties or foreign dynasties.
And all these can also be linked with the achievement system too. But I have to say I don't want to see an over-use of achievement-unlock elements.
Just some rough ideas.