The MP3, a three wheeled scooter is a pretty great bike.
I know it sounds weird, if it has two wheels up front, is it really a bike? The reviewers all agree that it is. It is automatic, comfortable, and nigh undroppable. Plus it can really corner. There are 250cc, 400cc, and 500cc versions (I think). I’ve seen a couple of them around town – thier owners love them.
Frankly, I don’t understand the hate-on many bikers have for scooters. A modern maxi-scoot (like the burgman 650) can cruise at 100mph just fine, and if you’re aggressive can handle pretty well to.
For a good giggle, read this: http://onewheeldrive.net/index.php?option=com_content&task=view&id=154&Itemid=124
Acidpope’s comment about boots is good too. Look at something like a Sidi off-road / adventure boot (like the ones worn on Long Way Round / Long Way Down). They provide a lot of ankle support. But since I don’t know what type of motion could upset your ankle, I’d speak with your doctor about it. (The sidi’s protect mostly against a fall breaking your leg, I’m not sure just what sort of stiffining they do to the ankle. Of course, you could always screw on a brace/bracket so the ankle stays fully rigid and you shift by moving your entire lower leg… just thinking aloud).
Lastly, about the weight of the cruiser – cruisers have very low centre of gravities. They are MUCH easier to keep upright / recover than a sportbike of a similar weight. A 450lbs cruiser takes no more effort to keep upright than a 330lbs Ninja 250 (speaking from personal experience).