It will probably be a while before this is an aftermarket option, but in my mind the clearly superior product would be some form of Hybrid aftermarket add on kit.

I mean, Toyota already has it down to a science where they can take basically the same Prius hybrid system and add it to any of their other models (Lexus rx400, Toyota Highlander).

Honestly, it shouldn't be that hard to make an electrical motor that adds boost to the engine and gets power from batteries and then recharges the batteries from the brakes. The torque would be immediate and strong! And it would pay for itself in gas savings.

Why install a supercharger or turbo that decreases efficiency when you could install and electric booster that adds power and fuel efficiency at the same time?

Anyways, I hope some enterprising guys get to work on an aftermarket hybrid kit ASAP. That's a turbo I'd pay for. A truly better product for the future. The equivalent of moving from floppy disks to CD's and dvd's IMHO.
Maybe someone with some connections could buy the same hybrid synergy drive toyota uses in the Prius and try to mod it to fit in the xB.

bump: good idea

The only issue I see with a Hybrid system on the xB is where do you put the battery packs? I think they would be able to get the rest of the hardware into the engine bay and put in the regenerative braking technology without much fuss but there just isn't a lot of room for the batteries that run the electric motor.

I would like to see a TDI package for the xB. You don't need the additional space for the batteries and the increase in fuel econmy would be a huge plus. VW makes a little 1.4L TDI. It only generates around 75 HP but it makes 144 lb-ft of torque and it gets fantastic gas mileage (around 60 mpg hwy)

They also have two 1.9L TDI engines. I'm not sure if a 1.9L would fit in an xB but the lower model 1.9L is rated at 100 hp at 177 lb-ft of torque and the higher end 1.9L is rated at 129 hp and 228 lb-ft of torque. The fuel economy numbers are in the high 40's to low 50's in hwy mpg for each of these engines. Not bad in any book.

It would be nice if the Hybrid could be a bolt-on, but it isn't; it is a completely different drivetrain, utilizing a CVT (planetary gear system), two electric motors and a regenerative braking system. Plus, the Prius Hybrid system is not the same in the Lexus or the Highlander. The Highlander will have a V6 and 270 horsepower AWD system, and it is nontraditional in the sense that there will be no driveshaft between the engine/motor combination and the rear wheels. (Just a set of electric leads going to a rear electric motor.)

You are right, though -- I have had a huge number of customers ask if the xB does come with the Hybrid Synergy Drive. We have been told that the corporate goal is to have a choice between a gasoline engine or HSD available on all product lines by 2010.