Not that any one service will win, but in order for any of them to do well, I think they need to do the following:
- Private messaging
- Group messages
- Images
- Free, or very very cheap (I don't think anybody has really tested the idea of charging a fee yet)
- Cross-platform - leaving out BBM and iMessage
- Some way of getting your address book from an existing address book
-- Web/desktop messaging. This isn't as important for the younger generation but for many others it is.
The only reason BBM and iMessage are or were popular was because they came pretty early and came with devices, but leaving friends out of conversations because they use a different platform from you is damaging, which is why WhatsApp became so popular, and why Facebook Messenger is so popular too.