I’m interested in data modeling and how tricky domains can be cleanly represented in code. These talks usually end up being my favorites at past elm confs 
I’m also interested in modeling and handling uncertainty at the edges of a system. Parsing third-party data, dealing with Maybe and Result, validating user input.
I want to learn more about forms. How to model them, handle per-field validations, keep related uncertainty about validity/presence of values out of the main model.
I want to hear about type safety. How can the compiler help me catch more bugs. How can I encode more business requirements into the type system?
To a lesser extent, I want to hear about compile performance. How can I write and structure code such that it compiles quickly?