These days most cars ARE fairly modular.

When the manufacturer introduces a new bodyshell, the running gear remains the same as the last model.

A year or two later the engines get an upgrade, and they are common to the whole range, so a 1.25 in a Ka is the same as a 1.25 in a Fiesta, (don't think Ka's come with a 2.0, but it is the same 2.0 in a Puma as in a Mondeo, etc...)

VW, SEAT and Skoda all use the same basic designs, just a slight tweak to the bodywork and spec, and a different badge.

There is just as much 're-use' going on with the smaller companies: the Tesla is based on a Lotus Elise(?), while Lotus design the suspension for half the european car makers. Buy a Caterham or similar, and only the chassis / bodyshell comes from them - the rest always comes from a larger manufacturer or two.

At a lower level still, the lights, wheels, switchgear, audio kit, batteries, alternators, starters, pumps and wiring all come from third-party suppliers, who naturally do their best to ensure the minimum possible number of variations.

Really, I don't see how things could sensibly get much more modular than they are now - quite the reverse: things like the Tesla need to be bleeding edge to have a chance, and that means that all of the novel stuff has to be custom-built.