You can swap battery, harddrive, memory, network chip yay. That was possible on nearly all laptops some years back. At the enterprise level both Dell and Lenovo have models that does most of this. (Unless recently discontinued)
Quite a few laptops had "bays" where you could swap out extra harddrive, cd rom, extra battery,
Then a while back computer had PCMCIA/PC Card, that let you add all sorts of very useful hardware.
All in all this is far less user configurable than a two bay, to pcmia slots. (with battery, memory easily upgraded).
I would love someone to come out with a laptop like that again.
Further modules for the framework are expensive, made by a single company, few variants. If the company fails you are stuck.
On the old style laptops, ram, memory, cd player, extra hard drive, batteries, PCMCIA/PC Cards were all usually available from several sources. Bays might be the most proprietary.