I like Heinleins solution in Starship Troopers. Only veterans vote. Everyone gets a chance to be a veteran.
I like this, but expand
Only those who have served under command,
for at least 3 years, and honorably discharged are Citizens
Depending on society's need at the time
This leaves open the CCC, and
“Peace Corp” types of service
But it must be under command,
meaningful, and honorable service
Also, if you are not a citizen,
the Government does not owe you a darn thing
Except equality of Rights under the Law
Make that two for the Heinlein proposal.
Israel uses that system I believe. 2 years of mandatory national service is required for everyone. If you aren’t up to IDF standards, there are non-military roles.