If you just power down by running the i2c slave power through a FET, either the i2c-slave might pull down the i2c lines in an uncontrolled fashion.. or the i2c-

If you just power down by running the i2c slave power through a FET, either the i2c-slave might pull down the i2c lines in an uncontrolled fashion.. or the i2c-slave might still be powered through the i2c SDA/SCL pullups. So if you really want to decouple a i2c slave for the purpose of resetting it, also run the i2c lines through a CMOS switch, which of course will complicate this even further. Then better don't use the buggy i2c device at all.