Category Theory after CatsNSheaves I’m currently reading MacLane’s “Categories for the Working Mathematician” and am loving it. So I bought a copy of Kashiwara & Schapira’s “Categories and Sheaves” and plan to tackle that next.
My question is, where can one go after Cats n Sheaves? More specifically, where in pure category theory? Is the only place to go higher category, or is there still a lot of interesting/important 1-category theory to learn?
If I come across overly ignorant, my apologies. 
 A: There's lots of other 1-dimensional category theory to study, although just as you can only get so far in algebra or topology without any category theory, eventually 2-category theory at the very least will inevitably begin to make its presence felt. 
On topos theory, which connects to logic but also to algebraic geometry: Sheaves in geometry and logic by Maclane & Moerdijk, Topoi: the categorial analysis of logic by Goldblatt. Goldblatt is easier and pitched more at the philosophically minded. 
Beyond a book on topos theory, the key intermediate book on category theory I'd suggest to anybody developing a more serious interest is Elements of Enriched Category Theory by Kelly. It's unfortunately the only book Kelly ever wrote, although dozens of his papers remain highly recommended. A great book on categories sufficiently more general than topoi to capture almost every single important large category is Locally Presentable and Accessible Categories by Adamek and Rosicky.
An area of application of topos theory which many find quite intriguing is synthetic differential geometry, which studies how to formalize calculus with a "real line" which contains infinitesimal elements, without destroying classical differential geometric results. Anders Kock has a couple of introductory-level books here. 
This is semi-secretly moving towards higher category theory, but Categorical Homotopy Theory by Riehl is a great introduction to another major application area, and works primarily with ordinary categories.  
Another direction to go in is homological algebra. Freyd's Abelian Categories and Gelfand-Manin's Triangulated Categories are nice introductions to certain aspects of this field. 
A somewhat off-the-beaten-path topoic is categorical universal algebra. For a treatment of a moderately generalized form of classical universal algebra from a categorical viewpoint, see Algebraic categories by Adamek, Rosicky, and Vitale; for an impressive categorical study of certain algebraic categories which had previously only been well understood in classical universal algebraic temrs, see Malt'sev, Homological, and Protomodular categories, by Borceux and Bourn, probably after studying abelian categories. 
Finally, while Categories and Sheaves is a good book, it's very difficult and would be quite a ways down my list of next books for you, unless you have a very specific reason to go in that direction. I also agree with BananaCats about Borceux. 
