I think that instead of focusing on interesting features or those that would look good on the box, doctest should instead focus on implementing features that people actually using doctest would benefit from on a frequent basis.
This is why I'm moving the roadmap into this issue, where everyone will be able to comment their thoughts!
This should mostly be a place to share rough ideas that are not concrete enough to justify an issue of their own and in general discuss the future development of doctest.

2.5:

• Clean up all the docs, benchmarks and whatnot
• Splitting the two monolithic doctest files up into more managable chunks (while keeping it single header for release)
• Generic container and STL stringification
• CaseInvariant string comparison utility
• Look into the feature of String. Full fledged lightweight std::string alternative (with more tests!), or removal in favor of it?

Stuff I will never get to but it would probably make sense to take a look at at some point:

• Make _LIBCPP_VERSION check independent from DOCTEST_CLANG #709 CI
• Fix clang in MSVC mode #705 CI
• Try fix NVCC #703 CI