VW Golf/Rabbit pickup is pretty light, for example. Challenges vs. rear engine are attaching appropriate half-shafts to drive the front wheels. If the smart transaxle is narrower than the stock donor car transaxle, you can center the Smart unit and run custom spacers that adapt the Smart transaxle to the Donor half-shafts. There are loads of small, cheap, lightweight FWD cars out there.
If you want something "completely different", try a (Ford Model) T-bucket roadster/pickup. You'll need to fabricate the rear subframe to hold the Smart transaxle/rear suspension, but everything else is available off the shelf. There's also the grand tradition of building it from whatever you can find cheap. Use the cavernous empty space under the hood as a "frunk" and/or to make people scratch their heads when you leave the hood up at a car show and there's nothing under there... Classic homebuilt registration could be easier, too, depending where you live.
You could also do a variant on the old VW-based Trike conversions. Use the Smart transaxle with a suitably sized (say, Honda Goldwing or so) front end. Not the best handling configuration as a tadpole trike, but generally straightforward to design and it registers as a motorcycle, making that end of things possibly a lot less hassle. Along similar lines, there's that XR3 3-wheel delta trike from R Q Riley that uses either a front wheel drive setup, a rear wheel drive setup, or both. They've also got a ton of other plans for lightweight vehicles. Lots more fab work, sure, but you end up with a different/unique vehicle.
You've got a really nice little engine package, have fun putting something neat around it!
