I don't think it's a good idea to use the same variable names + one other, since then the user is left with two different polynomial rings in n and n+1 variables, with no inclusion map between them, but such that the n variables of the first have the same names as the first n variables of the second. That looks like a cause for confusion!
I don't see why this is confusing, I think this is one of the cases that the coercion system implements nicely. To get the inclusion map is just:
sage: QQ['x, y, z'].coerce_map_from(QQ['x, y']) Call morphism: From: Multivariate Polynomial Ring in x, y over Rational Field To: Multivariate Polynomial Ring in x, y, z over Rational Field Nick
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