An M.2 SSD, like a Samsung 970 Pro. Or a SODIMM RAM stick. Or an M.2 Wi-Fi adapter.

While you're at it you can also screw the battery in instead of gluing it in place. Or screw the keyboard in instead of riveting it in place.

All of this is a reality in my Dell XPS 15. Keyboard is screwed in with 20-something tiny screws, it's firmly in place and it makes keyboard replacement a $20 affair that you can even do yourself should a bad day happen when you bump your drink onto it. The battery can be freshened up as well should you keep your laptop for more than a couple of years while using it daily, and it costs $80. Should your laptop fail completely you can salvage the data from it just by plugging the drive into another laptop or desktop (Apple removed a "lifeboat" connector from their 2018 MBP for example, not much was occupying its space though), you can upgrade the RAM fad down the line if your use case changer or OS becomes more demanding. Etc.

This is already possible and is happening right now, so Apple doing the Apple thing is not excusable by their laptops being thin or light, or portable or whatever.