it's a nice thought... but there are already so many tracking sites/software out there... i think unless it was something really special it would just be "another" tracking site. No offense of course!
Personally i currently prefer "pen and paper" method of logged to be honest! I can take it everwhere i go... and log things straight away like the carbs i eat and the insulin i take, which saves having to remember for when im at my computer next... which is difficult for me as my memory is... ummm... what the word.... oh yeh... rubbish
If you were to develop a site for tracking, havig said that It's nice to be able to do it whereever you are instead of having ot remember until your at a computer... is a WAP tracking site... so you can enter info via your mobile phone. That could possibly be more of a niché area for diabetes tracking sites.