Not OP, but as a Python developer with interests elsewhere, I'm very productive in modern C# with ASP.NET Core MVC. (And I can write it on a Mac and deploy on Linux, too!)

I'd go for libraries instead of frameworks. Frameworks tend to force you into using a specific pattern or architecture (e.g. active records for Django), and I think the programmer should have full control over that.

Having built couple of toy programs with OCaml, I'd be really interested in building a project in a functional language.

Not close to baked yet, but look forward to Rust. Hyper just got async built in, so the entire ecosystem is going to be scalability first from the start.

There are also a lot of "Flask-like" projects in Rust. Rocket for the framework, Tera for the templates, and Diesel for the ORM.

The barrier right now is mainly upstream. Rocket is a nightly-only Rust crate right now, and a lot of the UX features these kinds of frameworks need / want are still missing from the compiler (like async / await).