The basic problem is that the game treats every 'minor' the same, when 'minor's range from Canada in WW2 to Greece in WW1. Canada in WW2 was industrialised, mobilised, politically committed to the war, and fielded large ground and sea forces that were fully integrated into the 'major' in terms of equipment, doctrine and command. Indeed at times Canadians were regarded as an elite. Greece in WW1 was on the perpetual edge of bankruptcy and revolution, was a military liability and an economic disaster area (Britain spent half the war blockading southern Greece, starving the population, to try to prop up the King).
Currently there are very blunt tools to do this. Basically the force cap and the HQ quality are what there is to work with, and also the fact that minors don't benefit from Production Tech discounts. So you end up with situations like Bulgaria being shock troops or Romania adding half the MPP of the Russian Empire.
However there are other solutions than making every minor do its own research. I would suggest:
* Minors get an MPP modifier which can be set where required so you can prevent Romania in WW1 being the economic powerhouse of Russia, or Greece contributing a big convoy to Britain (note that majors already have MPP modifiers, Russia is set to 30% or something)
* Minors get a Readiness modifier which can be set per country to reflect differences in motivation, command, and training from their major. Bulgaria might get a -10% in both WW1 and WW2, for instance, to reflect the fact its troops will never be as good as Germans. Canada would have a 0 modifier, while Greece in WW1 might have -20%.
* It becomes possible to configure a tech limit on each Minor. So Bulgaria might for instance only have access to Infantry Weapons level 1 and Infantry Warfare level 2. (However this would have to be used cautiously, there's no reason why most majors couldn't have equipped minors as well as they wished, if they had unlimited resources - a Readiness penalty would already have a big impact).