I know this is serious, but maybe I misread your intention. The thing that makes Slack greater than all of the dominant IMs is that IM is their whole business.
Everyone else is busy "improving" gmail if you're Google, or monetizing voip/video chat, or (whatever it is that SalesForce does besides Chattr), meanwhile Slack is just chat, and whatever else you expected chat to encompass.
Yes. Instead of improving, they do voice, video, profile pages, silly pictures replacing your text and all that crap.
The only actual innovation I've seen to IM is ability to edit your last message.
Last people that tried to improve the actual IM chat were Google Wave but it crashed and burned because they fixated on everybody seeing as what everybody is writing when they write it (which was something nobody wanted).
Today, almost 20 years after ICQ people default to Skype even though it's closed crap for that purpose.
Any IM company that had some users had potential to become twitter or even facebook, but instead someone fresh came and stolen their lunch while they were playing with crappy voice and sponsored news feeds while accumulating bloat on their software.