Agreed, with above, and even Justin's idea. If you can use other major client's API's and work from there, that will give users a centralized location - hence making your IM used more widely due to the fact they don't have to use 5 IM clients, just yours.
Also, good luck on this project since it will contain LOTS of work including databases, and some far more advanced coding. 🙂
You should make it for windows, mac, linux, etc. and make games such as checkers, chess, etc. and make it so you can add other emails and chat with them such as Yahoo! Messenger, MSN, etc.
Also, keep in mind the actual major IM clients, they have a whole team that spends their whole job dedicated to creating those. So, you being an individual won't make anything elaborate at first, I don't expect you to, so good luck. Start out small and you can always work up from there.
- Multi-platform (Windows/Mac/Linux)
- Video chat
- Audio chat
- Avatars. Can be animated (GIF/Flash) and can also be uploaded from webcam.
- Multi-IM (like, Yahoo/Hotmail/AOL accounts all available)