I think that brigades available to be recruited from colonial provinces should be restricted to irregulars, because otherwise the UK and France are just too (ahistorically) powerful.
It makes sense, in my opinion, because colonies would not be able to provide the same level of military training as a proper military academy. Also, colonial populations were largely illiterate and didn't speak the same language as their European overlords, and so they wouldn't be as disciplined/organized as professional soldiers. And creating hordes of armor, airplanes, and other advanced units from colonial populations just doesn't make sense.
I'd really just like to see an end to the whole "create colonial empire=>build massive army=>world domination" thing, is what I'm saying.