I think an one important aspect of a platform vs something you can hack aftermarket is that the platform builder doesn't compete or hinder you as the aftermarket provider. In a way, for platforms, the aftermarket part builder is as much a customer as the people buying cars.

k00b

> ike if i wanted a bigger cabin for an extra row of seats, or a bigger bed for hauling stuff, i could just plug and play with differe

They pretty much have those options available now to chose from. 
In the back you have also several aftermarket brands that sell the cabin to mount

I was only thinking of the luxury addons that other companies could provide, but perhaps you could make the fundamentals of the cars more modular too.

tech

Toyota’s $10,000 Future Pickup Truck Is Basic Transportation Perfection

are you imagining a modular and open platform that other companies could build on?

like if i wanted a bigger cabin for an extra row of seats, or a bigger bed for hauling stuff, i could just plug and play with different parts?