I'm not really sure what you are asking so I don't know how to answer.
If you want to build an Announcements module I think that sounds great.
If you want advice about building it, I think first thing you must clarify what is an Announcement module.
What will it do that the Html Content module doesn't do?
What are the features of an Announcements module?
If I knew answers to these I might be able to make suggestions or ideas.