stanleyhuang:
The tough part is how to let the user download the compiled sketch (a HEX file) to Arduino board. Any suggestions will be welcomed.
This seems like the part that needs to be solved first.
I think you can create an incredibly useful web-based tool with the features you describe. It would probably be very attractive for new users. It certainly would nice to interactively help someone.
However, if downloading a HEX file and running a cryptic command is the last step, I doubt most will enjoy that step.