One big problem for people with diabetes is having to monitor their blood sugar. If it gets too high, there are massive problems, but if it gets too low, that's even worse. Severe diabetics must always carry around with them a syringe full of insulin in case of the first, and a candy bar in case of the second. They need to avoid sugar consumption when not having a low-sugar attack, and would benefit from weight loss.
So I propose a monitoring implant. Vampirically powered, it would constantly record the glucose concentration of the blood flowing past it, harvesting a tiny amount of that to power itself. Using RFID technology, the records could be downloaded into a wristwatch-like device for display or transfer into a computer. Alternatively, it could signal the person in case of excessively high or low readings by some means that I have yet to determine. A friend of mine says to put it in the watch, which would be smart because the watch could have more complex and sophisticated circuitry.
The glucose use of the machine would treat the underlying problem of uncontrolled glucose use, and the stored information would eliminate the need for the constant finger-prick tests that a diabetic person has to undergo as it is. Since it would read continuously, the person would have a record of their blood sugar levels through the day for further management. If the person has a good memory, this could lead to the identification of patterns that would lead to better health.
As a secondary benefit, if the owner is overweight, this may lead to a slight weight loss.