The subjects that are reasonably accessible after Calculus and
Linear Algebra are: - Real Analysis. - Abstract Algebra. -
Elementary Number Theory. - Elementary Set Theory. - Elementary
Logic. - Combinatorics and Discrete Mathematics. - Differential
Equations. Of these, 1-3 and 6 are "popular", so you want to
discard them. 8 is popular among "users" of mathematics
(engineers, for example). While Elementary Set Theory and
Elementary Logic will probably help with mathematics in general,
they are unlikely to give you any particular insight into either
calculus or linear algebra. The best possible insight into
calculus will be given by Real Analysis; the best possible
insight into Linear Algebra will be given by Abstract Algebra.
Elementary Number Theory is the source from which many other
areas of mathematics sprung, but it will give you no insight
into either linear algebra or into calculus (from Number Theory
and Real Analysis you can move on to Complex Analysis and
Analytic Number Theory, which will likely be