What specific features you love in a private messaging app?

I am an open source, web developer who created a private messaging web app hosted on Github. The web app can add friends and chat with them on a one-on-one basis like a bulletin board but privately. It's only text-based and you cannot group chat, but you can send a message to all of your friends on one send.

So, basically I am still trying to learn which features you would like to add on our messaging app since it is in a premature state. For example, photo sharing or file sharing, etc. Please explain why you wanted that feature and what feature you love on other messaging apps.

Thank you.