I think that's a reasonable amendment, but I'm not sure what this has to do with equal protection. The equal protection clause protects "all persons within [a state's] jurisdiction." Citizenship only limits the protection afforded by the privileges and immunities clause. (Also, the Due Process Clause is applicable to the states' treatment of all persons, not just citizens.)

This is just a small point about your title. I don't mean to imply that citizenship isn't important or anything.

That clause was inserted in 1868 to ensure that all freed slaves were in fact citizens.  I'm not quite sure about the status of American Indians at the time, but many were in fact held as slaves in California in the 1850s and perhaps 1860s.

I say keep it.  We don't have a particular ethnic or religious or ideological historical criterion for citizenship.  (Well, we probably should/would admit anyone of demonstrated majority descent from the American Indians aboriginal here.)   We have a behavioral criterion of positive social participation.  The loyalty oath new citizens have to give is to the Constitution, i.e. the social project and its people as a whole.

Furthermore, it's probably wise to be against anything Tom Tancredo is in favor of  :D