Unfortunately, with firearms registration and no-fly lists, Muslim registries are part of the slope we've set precedent for. A citizen should _never_ have to register with the government to exercise a freedom explicitly granted in the constitution.
Flying isn't a constitutionally protected right. Voting is though. Should you be able to vote without registering?
Correct.
The US Govt and many others already maintain a complete list of all persons, living and dead, including useful attributes like citizenship, current address, age (aka eligibility to vote), in real-time.
Maintaining separate, duplicate, opt-in, haphazardly maintained, patchwork voter registration databases is wasteful, error-prone, and only serves the purpose of disenfranchising undesirables.