Will Toyota build other plug-ins beyond the Prime?

The rear suspension isn't the problem - the Volkswagen products I'm referencing (Golf GTE/Audi A3 e-tron, and Passat GTE) have essentially an identical rear suspension design, and the Ioniq has a very similar one (might be identical, I haven't looked that closely).

The space for the mickey-mouse solar installation is the problem, but even with the NiMH buffer battery, it's still fairly space-inefficient under the seat.

VW and Hyundai's solution is to put the battery under the seat, with the fuel tank mounted up higher and to the rear. Because a weirdly-shaped fuel tank is relatively cheap and easy to make, whereas a weirdly-shaped battery is not (and, yes, their battery does have a bit of weirdness, but it's basically just splitting the pack into two, and running the exhaust between the two packs), their fuel tank is lower than the Prime's battery, and goes between the rear suspension:


I do suspect they have a different floor pan stamping to support this, but still...

 

But still, GTE/E-tron has smaller luggage space , yes I know it's a smaller car ... but more expensive one.

 

Sure, but the luggage space penalty compared to a regular Golf or A3 hatchback isn't that bad. (There is a penalty - it drops from 380 to 272 liters, VDA measurement, but I believe it's under-floor storage that you lose - visually, it looks the same.)

I suspect that a Prime packaged like the GTE/e-tron and Ioniq Plug-In would have a cargo floor like the spare tire-equipped Liftbacks (so, a flat load floor with the seats folded down), but with a fuel tank below the cargo floor instead of a spare tire. There might be some room for cargo behind the fuel tank, too, under the floor.