After switching to Meson by default in gh-16187 and closing the RFC for that switch in gh-13615, here is a summary of next steps to improve our build system support. Roughly in order of priority:

• BLD: pip install build issue with meson in Ubuntu virtualenv #16300
• Improved BLAS/LAPACK detection/support (it works now, but only using pkg-config), see Improve BLAS/LAPACK support: dependency detection, UX, and ILP64 #17244 and Feature: MKL dependency mesonbuild/meson#2835
• Meson complains about an absolute include path #16312
• Support editable installs (in progress): Add support for editable wheels mesonbuild/meson-python#47
• ILP64 support. This is now still tested in one numpy.distutils based CI job, and not supported yet in the Meson-based build. See Improve BLAS/LAPACK support: dependency detection, UX, and ILP64 #17244
• Passing through SciPy-specific build options when using pip, pypa/build or similar: Passing through project-specific config options mesonbuild/meson-python#54
• Improve Cython support in Meson (not so urgent for SciPy, but important when other projects like scikit-learn are going to move to Meson):
  • Depfile support: Cython dependency handling in .pxd and .pxi files mesonbuild/meson#9049
  • After that, fix Cython does not respect dependency for generating .pyx.c file mesonbuild/meson#8961 (will allow a much simpler use of Cython in meson.build files)
  • In Cython, we need to not depend on __init__.py's anymore, which can be done by merging this PR: MRG: add --module-name argument to cython command cython/cython#4548
• Documented and tested (in CI) support for cross-compilation. It may work now, but hasn't been tested yet. See Tracking issue for cross-compilation needs and issues #14812.
• Get rid of the annoying warnings that we now see in the configure stage of the build: Cannot avoid 'more than one output' warning from generator depends mesonbuild/meson#9837. Done in BLD: fix Meson build warnings about multiple targets #17882.
• Ensure our runtime lower bound on numpy is >= that of the numpy we built against: UX for constraining build-time and runtime dependencies mesonbuild/meson-python#29. Note that the numpy.distutils based build never did this, but it's a correctness issue that can pop up in practice. EDIT: fixed by design in numpy now that we're building against 2.0.0.dev0. Once 1.25.0 is our lowest supported version, this is completely fixed.
• Build warnings in wheel build phase about "purelib or platlib" #16310
• Making dependency('numpy') work in Meson: Adding numpy as a dependency with custom lookup  mesonbuild/meson#9598. This will for us only result in a minor cleanup in meson.build files, but it's nice to have for other projects that are adopting Meson - one less thing to figure out for them. EDIT: this is important for cross-compilation!
• Don't depend on a shebang line in the Meson build (BUG: Build failure due to problems with shebang line in cythoner.py #17020)