Hi Jon,
Interesting idea. Seems like it'd be a good novelty gift for my family to get me for example.
I guess you are already thinking this way, but it seems fairly natural to offer a birthday card, and maybe a range of other geek products around this.
On the product itself, it might be good to do alphabetical sudokus also - for 16x16 sudokus this could lead to some interesting message possibilities perhaps? Also, are there any other puzzles that lend themselves to this sort of customization - wordsearch perhaps?
I guess those are the two ways I'd consider expanding on an appealing starting idea.
Good luck, and let us know when you launch.
Best,
Ed.