Would be nice to see Rust's Option<T> and Result<T,E> ported over to C++ since I find that error handling extremely elegant compared to most C++ approaches.
Option has been around for a while as boost::optional or now std::experimental::optional. Result is coming and has a reference implementation as boost::expected. There's also a type-erased option type, "any".
They're quite nice, especially with a few trivial additions for ergonomics.