Might want to try Riot. It uses a protocol called Matrix, which is the successor to IRC. What this means is that you're not tied to them. Also the web, desktop, and mobile clients are pretty sweet.
Does Matrix really have the userbase in place to start calling it an IRC successor so definitively?
It may be aiming for it, and I do hope we see an open chat protocol succeed over all the Slacks and HipChats, but calling it the successor right now seems a little disingenuous!
Don't see anything else around that fits that bill. They also have amazing IRC bridging - I have been using Matrix bridged into Freenode as my primary 'IRC Client' for almost a year now.
