Making the HRE into the EU is the most obvious choice, it's perfect, electors, member states, etc.
NATO = USA (leader), Britain, Canada, Germany, Poland, Spain, Portugal, Italy, Iceland, Greece, Turkey, Hungary, Norway, Czech Republic, Denmark, France, Belgium, Luxembourg, and the Netherlands.
ANZUS = Australia (leader), New Zealand
China = China (leader), North Korea, Mongolia, Iran
US vassal states = Canada, Australia, Mexico, New Zealand (none of them do anything the US doesn't want them to do

)
Commonwealth of Independant States = Russia (leader), Azerbaijan, Armenia, Belarus, Georgia, Kazakhstan, Kyrgyzstan, Moldova, Tajikistan, Turkmenistan, Uzbekistan and Ukraine
League of Arab States = Egypt (leader), Algeria, Bahrain, Comoros, Djibouti, Iraq, Lebanon, Libya, Mauritania, Morocco, Oman, Palestine, Qatar, Saudi Arabia, Somalia, Sudan, Syria, Tunisia, UAE, Yemen
that's about all I can think of