These are not actually going to be much more customizable than other vehicles. The sole exception is in the infotainment department, where virtually all modern vehicles have some big overwrought thing that you probably don't want because it sucks, or it will in the future — which will affect you if you're the kind of person who keeps a vehicle. But if you are, the auto industry hates you.
A vehicle being "designed" to be customized is really entirely irrelevant because all of the same work has to be done in order to do that customization. There are "upfitter guides" for all of the vehicles you'd commonly expect to customize, especially vans and pickups, which help with that. I've used these to aid in doing solar installs in RV conversions. That is, any pickup you'd buy is already designed to be customized.
In order for there to be a significant aftermarket for parts for a vehicle, those vehicles have to be numerous. Then companies find it worthwhile to make seat install kits and such. Such things aren't generally complicated, so the modularity is really very overblown overall.
I do think the Slate concept has legs, but it's in the low price and ease of repair areas, and not at all in the customization department, which simply doesn't matter.