You should definitely consider the matrix protocol (https://matrix.org) that is used with riot (https://riot.im). Matrix allows for a complete decentralization and both the the protocol and the client are open source. There are even many other clients as matrix also works with for example weechat.

One of it's biggest strength are the support for bridges to other messengers like slack, irc and gitter. I personally haven't yet tried group calls or video chat, but considering that those also use end to end encryption just as messages and attachement and that this encryption has been fully audited, I could barely be any less hyped about all this. As the author of this article pointed out, all of skype's flaws are more or less inherent of big centralized messengers and this is exactly what we get around with matrix. It's really amazing in what a great state it already is. Sure there is still lots of work left, but to me it appears to be the currently most promising project.