Here is an idea I thought long and hard about for the last 3 seconds....

Say one, rather than making the entire phone modular, adds just one cartridge slot. Have it span the bottom half of the back of the phone and be a few mm deep. Cartridges can have 4 form factors. 1) flush with the back of the phone. 2) stick out from the back. 3) increase thickness of the entire phone. Or 4) like 3 but comes with the same slot as the phone so that one can stack cartridges.

The first base phone should be functional by it self but have really low specs. A slow cpu, little memory, little storage, small battery. It may even run on android and have a ton of preloaded apps no one wants. Ideally the most expensive component should be the cartridge connector.

And then, here it comes, you've already guessed it! The entire linux computer goes on the cartridge.

Have a similar dock that turns the cartridge into a desktop computer and a dock that connects it to your PC.

Software development would be glorious.

In the initial demo it should run Windows! This will send a strong signal to other otherwise uninterested parties that this is a real computer... finally...

While official builds should probably exist let other vendors go wild building their own proprietary closed source cartridges.

There should be infinite possibilities. People will make things we cant imagine. Stuff we will never see on flagship phones because 99% doesn't need it.

Some might simply but badly want usb ports.

Stupid example: I have a digital camera, I have to plug it into a computer and do all kinds of things before they may appear on my server, like booting the machine, opening apps and figuring out where the hell folders are. The pictures are great but not that much better than my phone which can conveniently send them places. But what I really need is to just plug in the camera and have the technology figure out which are the new images and upload them. It should require zero screen time.

The next guy might want an ethernet port, hdmi, serial, scan barcodes by pressing a real button that also unlocks and opens the correct app. You might even have a bulky cartridge that prints receipts. A large antenna and/or a week worth of battery. I'm not at all sure if people want it but a cassette player would be possible. A boom box with atx drive bays. etc etc

Then when you buy the next generation or are bored playing with it, the screen is cracked and the battery is worn out you turn it into a security camera that works when the power is cut and can send [picture] sms, make phone calls and play threatening messages to intruders.