The ‘I’ gives direct rise to two letters—the ‘H’ and the ‘J’. Both these letters are incredibly straightforward to design.
The ‘H’ is just two capital ‘I’s connected by a crossbar the width of one of the arms on the ‘Z’. The letter, not including serifs, should be about as wide as the ‘Z’ as well, and the crossbar should be located slightly above the letter’s median. As usual, the interior serifs on the letter’s two stems should be shortened slightly.
The ‘J’ is even simpler to design, though it contains much more creative choice. The curved tail can extend all the way to the descender line, stop at the baseline, or terminate anywhere in between. It can also be blobbed or end in a finial.
Very often, the letter’s lower half is identical to that of the lowercase ‘j’, weight adjusted of course. You may wish to shorten the right side of its top serif since it is alone and points away from the direction that the letter opens up to.